diff --git a/src/hydra-queue-runner/build-remote.cc b/src/hydra-queue-runner/build-remote.cc
index a0408f55..cf4de781 100644
--- a/src/hydra-queue-runner/build-remote.cc
+++ b/src/hydra-queue-runner/build-remote.cc
@@ -116,7 +116,7 @@ static void copyClosureTo(ref<Store> destStore,
 
 void State::buildRemote(ref<Store> destStore,
     Machine::ptr machine, Step::ptr step,
-    unsigned int maxSilentTime, unsigned int buildTimeout,
+    unsigned int maxSilentTime, unsigned int buildTimeout, unsigned int repeats,
     RemoteResult & result, std::shared_ptr<ActiveStep> activeStep)
 {
     assert(BuildResult::TimedOut == 8);
@@ -263,9 +263,10 @@ void State::buildRemote(ref<Store> destStore,
         to << maxSilentTime << buildTimeout;
         if (GET_PROTOCOL_MINOR(remoteVersion) >= 2)
             to << 64 * 1024 * 1024; // == maxLogSize
-        if (GET_PROTOCOL_MINOR(remoteVersion) >= 3)
-            // FIXME: make the number of repeats configurable.
-            to << (step->isDeterministic ? 1 : 0);
+        if (GET_PROTOCOL_MINOR(remoteVersion) >= 3) {
+            to << repeats // == build-repeat
+               << step->isDeterministic; // == enforce-determinism
+        }
         to.flush();
 
         result.startTime = time(0);
@@ -295,6 +296,10 @@ void State::buildRemote(ref<Store> destStore,
             result.stepStatus = bsSuccess;
         } else {
             result.errorMsg = readString(from);
+            if (GET_PROTOCOL_MINOR(remoteVersion) >= 3) {
+                result.timesBuilt = readInt(from);
+                result.isNonDeterministic = readInt(from);
+            }
             switch ((BuildResult::Status) res) {
                 case BuildResult::Built:
                     result.stepStatus = bsSuccess;
diff --git a/src/hydra-queue-runner/builder.cc b/src/hydra-queue-runner/builder.cc
index 4e8f6cf3..667a5192 100644
--- a/src/hydra-queue-runner/builder.cc
+++ b/src/hydra-queue-runner/builder.cc
@@ -86,6 +86,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
     BuildID buildId;
     Path buildDrvPath;
     unsigned int maxSilentTime, buildTimeout;
+    unsigned int repeats = step->isDeterministic ? 1 : 0;
 
     {
         std::set<Build::ptr> dependents;
@@ -113,6 +114,11 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
               build = build2;
               enqueueNotificationItem({NotificationItem::Type::BuildStarted, build->id});
             }
+            {
+                auto i = jobsetRepeats.find(std::make_pair(build2->projectName, build2->jobsetName));
+                if (i != jobsetRepeats.end())
+                    repeats = std::max(repeats, i->second);
+            }
         }
         if (!build) build = *dependents.begin();
 
@@ -121,8 +127,8 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
         maxSilentTime = build->maxSilentTime;
         buildTimeout = build->buildTimeout;
 
-        printMsg(lvlInfo, format("performing step ‘%1%’ on ‘%2%’ (needed by build %3% and %4% others)")
-            % step->drvPath % machine->sshName % buildId % (dependents.size() - 1));
+        printInfo("performing step ‘%s’ %d times on ‘%s’ (needed by build %d and %d others)",
+            step->drvPath, repeats + 1, machine->sshName, buildId, (dependents.size() - 1));
     }
 
     bool quit = buildId == buildOne && step->drvPath == buildDrvPath;
@@ -162,7 +168,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
         /* Do the build. */
         try {
             /* FIXME: referring builds may have conflicting timeouts. */
-            buildRemote(destStore, machine, step, maxSilentTime, buildTimeout, result, activeStep);
+            buildRemote(destStore, machine, step, maxSilentTime, buildTimeout, repeats, result, activeStep);
         } catch (NoTokens & e) {
             result.stepStatus = bsNarSizeLimitExceeded;
         } catch (Error & e) {
@@ -224,8 +230,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
             auto mc = startDbUpdate();
             {
             pqxx::work txn(*conn);
-            finishBuildStep(txn, result.startTime, result.stopTime, result.overhead, buildId,
-                stepNr, machine->sshName, result.stepStatus, result.errorMsg);
+            finishBuildStep(txn, result, buildId, stepNr, machine->sshName);
             txn.commit();
             }
             stepFinished = true;
@@ -279,8 +284,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
 
                 pqxx::work txn(*conn);
 
-                finishBuildStep(txn, result.startTime, result.stopTime, result.overhead,
-                    buildId, stepNr, machine->sshName, bsSuccess);
+                finishBuildStep(txn, result, buildId, stepNr, machine->sshName);
 
                 for (auto & b : direct) {
                     printMsg(lvlInfo, format("marking build %1% as succeeded") % b->id);
@@ -386,8 +390,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
 
                 if (result.stepStatus != bsCachedFailure && !stepFinished) {
                     assert(stepNr);
-                    finishBuildStep(txn, result.startTime, result.stopTime, result.overhead,
-                        buildId, stepNr, machine->sshName, result.stepStatus, result.errorMsg);
+                    finishBuildStep(txn, result, buildId, stepNr, machine->sshName);
                 }
 
                 /* Mark all builds that depend on this derivation as failed. */
diff --git a/src/hydra-queue-runner/hydra-queue-runner.cc b/src/hydra-queue-runner/hydra-queue-runner.cc
index 8099292d..a531714a 100644
--- a/src/hydra-queue-runner/hydra-queue-runner.cc
+++ b/src/hydra-queue-runner/hydra-queue-runner.cc
@@ -264,20 +264,21 @@ unsigned int State::createBuildStep(pqxx::work & txn, time_t startTime, BuildID
 }
 
 
-void State::finishBuildStep(pqxx::work & txn, time_t startTime, time_t stopTime, unsigned int overhead,
-    BuildID buildId, unsigned int stepNr, const std::string & machine, BuildStatus status,
-    const std::string & errorMsg, BuildID propagatedFrom)
+void State::finishBuildStep(pqxx::work & txn, const RemoteResult & result,
+    BuildID buildId, unsigned int stepNr, const std::string & machine)
 {
-    assert(startTime);
-    assert(stopTime);
+    assert(result.startTime);
+    assert(result.stopTime);
     txn.parameterized
-        ("update BuildSteps set busy = 0, status = $1, propagatedFrom = $4, errorMsg = $5, startTime = $6, stopTime = $7, machine = $8, overhead = $9 where build = $2 and stepnr = $3")
-        ((int) status)(buildId)(stepNr)
-        (propagatedFrom, propagatedFrom != 0)
-        (errorMsg, errorMsg != "")
-        (startTime)(stopTime)
+        ("update BuildSteps set busy = 0, status = $1, errorMsg = $4, startTime = $5, stopTime = $6, machine = $7, overhead = $8, timesBuilt = $9, isNonDeterministic = $10 where build = $2 and stepnr = $3")
+        ((int) result.stepStatus)(buildId)(stepNr)
+        (result.errorMsg, result.errorMsg != "")
+        (result.startTime)(result.stopTime)
         (machine, machine != "")
-        (overhead, overhead != 0).exec();
+        (result.overhead, result.overhead != 0)
+        (result.timesBuilt, result.timesBuilt > 0)
+        (result.isNonDeterministic, result.timesBuilt > 1)
+        .exec();
 }
 
 
@@ -809,6 +810,13 @@ void State::run(BuildID buildOne)
 
     useSubstitutes = isTrue(hydraConfig["use-substitutes"]);
 
+    // FIXME: hacky mechanism for configuring determinism checks.
+    for (auto & s : tokenizeString<Strings>(hydraConfig["xxx-jobset-repeats"])) {
+        auto s2 = tokenizeString<std::vector<std::string>>(s, ":");
+        if (s2.size() != 3) throw Error("bad value in xxx-jobset-repeats");
+        jobsetRepeats.emplace(std::make_pair(s2[0], s2[1]), std::stoi(s2[2]));
+    }
+
     {
         auto conn(dbPool.get());
         clearBusy(*conn, 0);
diff --git a/src/hydra-queue-runner/state.hh b/src/hydra-queue-runner/state.hh
index 2994b9fb..4c23471b 100644
--- a/src/hydra-queue-runner/state.hh
+++ b/src/hydra-queue-runner/state.hh
@@ -48,6 +48,9 @@ struct RemoteResult
     bool canCache = false; // for bsFailed
     std::string errorMsg; // for bsAborted
 
+    unsigned int timesBuilt = 0;
+    bool isNonDeterministic = false;
+
     time_t startTime = 0, stopTime = 0;
     unsigned int overhead = 0;
     nix::Path logFile;
@@ -414,6 +417,10 @@ private:
        from showing up as busy until the queue runner is restarted. */
     nix::Sync<std::set<std::pair<BuildID, int>>> orphanedSteps;
 
+    /* How often the build steps of a jobset should be repeated in
+       order to detect non-determinism. */
+    std::map<std::pair<std::string, std::string>, unsigned int> jobsetRepeats;
+
 public:
     State();
 
@@ -437,10 +444,8 @@ private:
         const std::string & machine, BuildStatus status, const std::string & errorMsg = "",
         BuildID propagatedFrom = 0);
 
-    void finishBuildStep(pqxx::work & txn, time_t startTime, time_t stopTime,
-        unsigned int overhead, BuildID buildId, unsigned int stepNr,
-        const std::string & machine, BuildStatus status, const std::string & errorMsg = "",
-        BuildID propagatedFrom = 0);
+    void finishBuildStep(pqxx::work & txn, const RemoteResult & result, BuildID buildId, unsigned int stepNr,
+        const std::string & machine);
 
     int createSubstitutionStep(pqxx::work & txn, time_t startTime, time_t stopTime,
         Build::ptr build, const nix::Path & drvPath, const std::string & outputName, const nix::Path & storePath);
@@ -492,6 +497,7 @@ private:
     void buildRemote(nix::ref<nix::Store> destStore,
         Machine::ptr machine, Step::ptr step,
         unsigned int maxSilentTime, unsigned int buildTimeout,
+        unsigned int repeats,
         RemoteResult & result, std::shared_ptr<ActiveStep> activeStep);
 
     void markSucceededBuild(pqxx::work & txn, Build::ptr build,
diff --git a/src/lib/Hydra/Schema/BuildSteps.pm b/src/lib/Hydra/Schema/BuildSteps.pm
index aeb04a8f..bf572fcf 100644
--- a/src/lib/Hydra/Schema/BuildSteps.pm
+++ b/src/lib/Hydra/Schema/BuildSteps.pm
@@ -103,6 +103,16 @@ __PACKAGE__->table("BuildSteps");
   data_type: 'integer'
   is_nullable: 1
 
+=head2 timesbuilt
+
+  data_type: 'integer'
+  is_nullable: 1
+
+=head2 isnondeterministic
+
+  data_type: 'boolean'
+  is_nullable: 1
+
 =cut
 
 __PACKAGE__->add_columns(
@@ -132,6 +142,10 @@ __PACKAGE__->add_columns(
   { data_type => "integer", is_foreign_key => 1, is_nullable => 1 },
   "overhead",
   { data_type => "integer", is_nullable => 1 },
+  "timesbuilt",
+  { data_type => "integer", is_nullable => 1 },
+  "isnondeterministic",
+  { data_type => "boolean", is_nullable => 1 },
 );
 
 =head1 PRIMARY KEY
@@ -201,8 +215,8 @@ __PACKAGE__->belongs_to(
 );
 
 
-# Created by DBIx::Class::Schema::Loader v0.07043 @ 2016-02-16 18:04:52
-# DO NOT MODIFY THIS OR ANYTHING ABOVE! md5sum:TRALbEoaF/OIOyERYCyxkw
+# Created by DBIx::Class::Schema::Loader v0.07045 @ 2016-12-07 13:48:19
+# DO NOT MODIFY THIS OR ANYTHING ABOVE! md5sum:3FYkqSUfgWmiqZzmX8J4TA
 
 my %hint = (
     columns => [
diff --git a/src/root/build.tt b/src/root/build.tt
index e78fc883..f9cffc0f 100644
--- a/src/root/build.tt
+++ b/src/root/build.tt
@@ -50,13 +50,20 @@ FOR step IN steps; IF step.busy; busy = 1; END; END;
                  END %]
             </td>
             <td>[% IF step.busy == 1 || ((step.machine || step.starttime) && (step.status == 0 || step.status == 1 || step.status == 3 || step.status == 4 || step.status == 7)); INCLUDE renderMachineName machine=step.machine; ELSE; "<em>n/a</em>"; END %]</td>
-            <td>
+            <td class="step-status">
               [% IF step.busy == 1 %]
                 <strong>Building</strong>
               [% ELSIF step.status == 0 %]
-                Succeeded
+                [% IF step.isnondeterministic %]
+                  <span class="warn">Succeeded with non-determistic result</span>
+                [% ELSE %]
+                  Succeeded
+                [% END %]
+                [% IF step.timesbuilt > 1 %]
+                  ([% step.timesbuilt %] times)
+                [% END %]
               [% ELSIF step.status == 3 %]
-                <span class="error"><strong>Aborted</strong>[% IF step.errormsg %]: [% HTML.escape(step.errormsg); END %]</span>
+                <span class="error">Aborted</span>[% IF step.errormsg %]: <em>[% HTML.escape(step.errormsg) %]</em>[% END %]
               [% ELSIF step.status == 4 %]
                 <span class="error">Cancelled</span>
               [% ELSIF step.status == 7 %]
@@ -70,9 +77,9 @@ FOR step IN steps; IF step.busy; busy = 1; END; END;
               [% ELSIF step.status == 11 %]
                 <span class="error">Output limit exceeded</span>
               [% ELSIF step.status == 12 %]
-                <span class="error">Non-deterministic build</span>
+                <span class="error">Non-determinism detected</span> [% IF step.timesbuilt %] after [% step.timesbuilt %] times[% END %]
               [% ELSIF step.errormsg %]
-                <span class="error">Failed: [% HTML.escape(step.errormsg) %]</span>
+                <span class="error">Failed</span>: <em>[% HTML.escape(step.errormsg) %]</em>
               [% ELSE %]
                 <span class="error">Failed</span>
               [% END %]
@@ -137,7 +144,7 @@ FOR step IN steps; IF step.busy; busy = 1; END; END;
     <table>
       <tr>
         <td>
-          [% INCLUDE renderBuildStatusIcon size=128, build=build %]
+          [% INCLUDE renderBuildStatusIcon size=128 build=build %]
         </td>
         <td>
           <table class="info-table">
diff --git a/src/root/static/css/hydra.css b/src/root/static/css/hydra.css
index b8d9d7b4..f4cf1d8b 100644
--- a/src/root/static/css/hydra.css
+++ b/src/root/static/css/hydra.css
@@ -132,3 +132,13 @@ div.flot-tooltip {
     opacity: 0.80;
     z-index: 100;
 }
+
+td.step-status span.error {
+    color: red;
+    font-weight: bold;
+}
+
+td.step-status span.warn {
+    color: #aaaa00;
+    font-weight: bold;
+}
diff --git a/src/sql/hydra.sql b/src/sql/hydra.sql
index fc20e069..e8f958b1 100644
--- a/src/sql/hydra.sql
+++ b/src/sql/hydra.sql
@@ -289,6 +289,12 @@ create table BuildSteps (
     -- Time in milliseconds spend copying stuff from/to build machines.
     overhead      integer,
 
+    -- How many times this build step was done (for checking determinism).
+    timesBuilt    integer,
+
+    -- Whether this build step produced different results when repeated.
+    isNonDeterministic boolean,
+
     primary key   (build, stepnr),
     foreign key   (build) references Builds(id) on delete cascade,
     foreign key   (propagatedFrom) references Builds(id) on delete cascade
diff --git a/src/sql/upgrade-52.sql b/src/sql/upgrade-52.sql
new file mode 100644
index 00000000..ac376df6
--- /dev/null
+++ b/src/sql/upgrade-52.sql
@@ -0,0 +1,3 @@
+alter table BuildSteps
+  add column timesBuilt integer,
+  add column isNonDeterministic boolean;