diff --git a/src/main.cpp b/src/main.cpp
index 847b1ea8a61..874769c483b 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -362,6 +362,14 @@ bool CTransaction::IsStandard() const
     if (!IsFinal())
         return false;
 
+    // Extremely large transactions with lots of inputs can cost the network
+    // almost as much to process as they cost the sender in fees, because
+    // computing signature hashes is O(ninputs*txsize). Limiting transactions
+    // to MAX_STANDARD_TX_SIZE mitigates CPU exhaustion attacks.
+    unsigned int sz = this->GetSerializeSize(SER_NETWORK, CTransaction::CURRENT_VERSION);
+    if (sz >= MAX_STANDARD_TX_SIZE)
+        return false;
+
     BOOST_FOREACH(const CTxIn& txin, vin)
     {
         // Biggest 'standard' txin is a 3-signature 3-of-3 CHECKMULTISIG
diff --git a/src/main.h b/src/main.h
index 23a4d3ba318..d8e6a1bd33a 100644
--- a/src/main.h
+++ b/src/main.h
@@ -28,6 +28,8 @@ struct CBlockIndexWorkComparator;
 static const unsigned int MAX_BLOCK_SIZE = 1000000;
 /** The maximum size for mined blocks */
 static const unsigned int MAX_BLOCK_SIZE_GEN = MAX_BLOCK_SIZE/2;
+/** The maximum size for transactions we're willing to relay/mine **/
+static const unsigned int MAX_STANDARD_TX_SIZE = MAX_BLOCK_SIZE_GEN/5;
 /** The maximum allowed number of signature check operations in a block (network rule) */
 static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
 /** The maximum number of orphan transactions kept in memory */
diff --git a/src/wallet.cpp b/src/wallet.cpp
index b8ef2a20bfc..2317ac31ac7 100644
--- a/src/wallet.cpp
+++ b/src/wallet.cpp
@@ -1208,7 +1208,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64> >& vecSend, CW
 
                 // Limit size
                 unsigned int nBytes = ::GetSerializeSize(*(CTransaction*)&wtxNew, SER_NETWORK, PROTOCOL_VERSION);
-                if (nBytes >= MAX_BLOCK_SIZE_GEN/5)
+                if (nBytes >= MAX_STANDARD_TX_SIZE)
                     return false;
                 dPriority /= nBytes;