diff --git a/main.cpp b/main.cpp
index 5cb1342b..4214f27f 100644
--- a/main.cpp
+++ b/main.cpp
@@ -739,21 +739,28 @@ bool CTransaction::AcceptToMemoryPool(CTxDB& txdb, bool fCheckInputs, bool* pfMi
             return error("AcceptToMemoryPool() : not enough fees");
 
         // Continuously rate-limit free transactions
+        // This mitigates 'penny-flooding' -- sending thousands of free transactions just to
+        // be annoying or make other's transactions take longer to confirm.
         if (nFees < CENT)
         {
+            static CCriticalSection cs;
             static double dFreeCount;
             static int64 nLastTime;
             int64 nNow = GetTime();
-            // Use an exponentially decaying ~10-minute window:
-            dFreeCount *= pow(1.0 - 1.0/600.0, (double)(nNow - nLastTime));
-            nLastTime = nNow;
-            // -limitfreerelay unit is thousand-bytes-per-minute
-            // At default rate it would take over a month to fill 1GB
-            if (dFreeCount > GetArg("-limitfreerelay", 15)*10*1000 && !IsFromMe())
-                return error("AcceptToMemoryPool() : free transaction rejected by rate limiter");
-            if (fDebug)
-                printf("Rate limit dFreeCount: %g => %g\n", dFreeCount, dFreeCount+nSize);
-            dFreeCount += nSize;
+
+            CRITICAL_BLOCK(cs)
+            {
+                // Use an exponentially decaying ~10-minute window:
+                dFreeCount *= pow(1.0 - 1.0/600.0, (double)(nNow - nLastTime));
+                nLastTime = nNow;
+                // -limitfreerelay unit is thousand-bytes-per-minute
+                // At default rate it would take over a month to fill 1GB
+                if (dFreeCount > GetArg("-limitfreerelay", 15)*10*1000 && !IsFromMe())
+                    return error("AcceptToMemoryPool() : free transaction rejected by rate limiter");
+                if (fDebug)
+                    printf("Rate limit dFreeCount: %g => %g\n", dFreeCount, dFreeCount+nSize);
+                dFreeCount += nSize;
+            }
         }
     }