diff --git a/CHANGES b/CHANGES
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,8 @@
+1.2.1
+
+- Fix concurrency bug in Data.UUID.V1 (thanks to Neil Mitchell for reporting
+and a test case)
+
 1.2.0
   (Contributors: Antoine Latter & Mark Lentczner)
 
diff --git a/Data/UUID/V1.hs b/Data/UUID/V1.hs
--- a/Data/UUID/V1.hs
+++ b/Data/UUID/V1.hs
@@ -21,7 +21,7 @@
 import Data.Bits
 import Data.Word
 
-import Data.IORef
+import Control.Concurrent.MVar
 import System.IO
 import System.IO.Unsafe
 
@@ -65,29 +65,27 @@
 -- the clock value on initialization.
 stepTime :: IO (Maybe (Word16, Word64))
 stepTime = do
-  State c0 h0 <- readIORef state
   h1 <- fmap hundredsOfNanosSinceGregorianReform getCurrentTime
-  if h1 > h0
-    then  do
-      writeIORef state $ State c0 h1
-      return $ Just (c0, h1)
-    else  do
+  modifyMVar state $ \s@(State c0 h0) ->
+   if h1 > h0
+    then
+      return (State c0 h1, Just (c0, h1))
+    else
       let
         c1 = succ c0
-      if c1 <= 0x3fff -- when clock is initially randomized,
+      in if c1 <= 0x3fff -- when clock is initially randomized,
                       -- then this test will need to change
-        then  do
-          writeIORef state $ State c1 h1
-          return $ Just (c1, h1)
-        else  do
-          return Nothing
+         then
+          return (State c1 h1, Just (c1, h1))
+        else
+          return (s, Nothing)
 
 
 {-# NOINLINE state #-}
-state :: IORef State
+state :: MVar State
 state = unsafePerformIO $ do
   h0 <- fmap hundredsOfNanosSinceGregorianReform getCurrentTime
-  newIORef $ State 0 h0 -- the 0 should be a random number
+  newMVar $ State 0 h0 -- the 0 should be a random number
 
 
 data State = State
diff --git a/tests/BenchUUID.hs b/tests/BenchUUID.hs
--- a/tests/BenchUUID.hs
+++ b/tests/BenchUUID.hs
@@ -1,6 +1,7 @@
 import Control.Parallel.Strategies
 import Criterion.Main
 import Data.Char (ord)
+import Data.IORef
 import Data.Maybe (fromJust)
 import Data.Word
 import qualified Data.Set as Set
@@ -11,18 +12,15 @@
 import qualified Data.UUID.V3 as U3
 import qualified Data.UUID.V5 as U5
 import System.Random
-
+import System.Random.Mersenne.Pure64
 
 instance NFData BL.ByteString where
     rnf BL.Empty        = ()
     rnf (BL.Chunk _ ts) = rnf ts
 
 instance NFData U.UUID where
-    rnf u = ()
 
 
-
-
 main :: IO ()
 main = do
         u1 <- randomIO
@@ -36,6 +34,15 @@
                         BL.pack [0x16, 0x9a, 0x5a, 0x43, 0xc0, 0x51, 0x4a, 0x16,
                                  0x98, 0xf4, 0x08, 0x44, 0x7d, 0xdd, 0x5d, 0xc0]
             u3  = fromJust $ U.fromString "dea6f619-1038-438b-b4af-f1cdec1e6e23"
+
+        -- setup for random generation
+        randomState <- newPureMT >>= newIORef
+        let randomUUID = do
+              state <- readIORef randomState
+              let (uuid, state') = random state
+              writeIORef randomState state'
+              return uuid
+
         defaultMain [
             bgroup "testing" [
                 bench "null non-nil"   $ whnf U.null u1,
@@ -52,7 +59,7 @@
                 ],
             bgroup "generation" [
                 bench "V1" $ nfIO U.nextUUID,
-                bench "V4" $ nfIO (randomIO :: IO U.UUID),
+                bench "V4" $ nfIO (randomUUID :: IO U.UUID),
                 bench "V3" $ nf   (U3.generateNamed U3.namespaceURL) n1,
                 bench "V5" $ nf   (U5.generateNamed U5.namespaceURL) n1
                 ],
diff --git a/uuid.cabal b/uuid.cabal
--- a/uuid.cabal
+++ b/uuid.cabal
@@ -1,5 +1,5 @@
 Name: uuid
-Version: 1.2.0
+Version: 1.2.1
 Copyright: (c) 2008-2009 Antoine Latter
 Author: Antoine Latter
 Maintainer: aslatter@gmail.com
@@ -56,7 +56,8 @@
   Buildable: True
   Build-Depends: random, binary, bytestring, Crypto, maccatcher,
                  time, base >=3, base < 5, containers,
-                 criterion >= 0.3 && < 0.5, parallel >= 2.1 && < 2.3
+                 criterion >= 0.3 && < 0.5, parallel >= 2.1 && < 2.3,
+                 mersenne-random-pure64 >= 0.2 && < 0.3
  Else
   Buildable: False