diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index fbd618d41..0505c5b24 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1058,6 +1058,7 @@ void EvalState::printStats()
     printMsg(v, format("  stack space per eval() level: %1% bytes") % ((&x - deepestStack) / (float) maxRecursionDepth));
     printMsg(v, format("  values allocated: %1%") % nrValues);
     printMsg(v, format("  environments allocated: %1%") % nrEnvs);
+    printMsg(v, format("  symbols in symbol table: %1%") % symbols.size());
 }
 
 
diff --git a/src/libexpr/symbol-table.hh b/src/libexpr/symbol-table.hh
index ae0751d11..424c23538 100644
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@@ -2,6 +2,7 @@
 #define __SYMBOL_TABLE_H
 
 #include <map>
+#include <tr1/unordered_set>
 
 #include "types.hh"
 
@@ -59,7 +60,7 @@ inline std::ostream & operator << (std::ostream & str, const Symbol & sym)
 class SymbolTable
 {
 private:
-    typedef std::set<string> Symbols;
+    typedef std::tr1::unordered_set<string> Symbols;
     Symbols symbols;
 
 public:
@@ -68,6 +69,11 @@ public:
         std::pair<Symbols::iterator, bool> res = symbols.insert(s);
         return Symbol(&*res.first);
     }
+
+    unsigned int size() const
+    {
+        return symbols.size();
+    }
 };
 
 }