diff --git a/src/whoosh/util/cache.py b/src/whoosh/util/cache.py
--- a/src/whoosh/util/cache.py
+++ b/src/whoosh/util/cache.py
@@ -80,33 +80,37 @@
         stats = [0, 0]  # Hits, misses
         data = {}
         lastused = {}
+        lock = Lock()

         @functools.wraps(user_function)
         def wrapper(*args):
-            try:
-                result = data[args]
-                stats[0] += 1  # Hit
-            except KeyError:
-                stats[1] += 1  # Miss
-                if len(data) == maxsize:
-                    for k, _ in nsmallest(maxsize // 10 or 1,
-                                          iteritems(lastused),
-                                          key=itemgetter(1)):
-                        del data[k]
-                        del lastused[k]
-                data[args] = user_function(*args)
-                result = data[args]
-            finally:
-                lastused[args] = time()
-            return result
+            with lock:
+                try:
+                    result = data[args]
+                    stats[0] += 1  # Hit
+                except KeyError:
+                    stats[1] += 1  # Miss
+                    if len(data) == maxsize:
+                        for k, _ in nsmallest(maxsize // 10 or 1,
+                                              iteritems(lastused),
+                                              key=itemgetter(1)):
+                            del data[k]
+                            del lastused[k]
+                    data[args] = user_function(*args)
+                    result = data[args]
+                finally:
+                    lastused[args] = time()
+                return result

         def cache_info():
-            return stats[0], stats[1], maxsize, len(data)
+            with lock:
+                return stats[0], stats[1], maxsize, len(data)

         def cache_clear():
-            data.clear()
-            lastused.clear()
-            stats[0] = stats[1] = 0
+            with lock:
+                data.clear()
+                lastused.clear()
+                stats[0] = stats[1] = 0

         wrapper.cache_info = cache_info
         wrapper.cache_clear = cache_clear
@@ -182,29 +186,33 @@
     def decorating_function(user_function):
         stats = [0, 0]  # hits, misses
         data = {}
+        lock = Lock()

         @functools.wraps(user_function)
         def wrapper(*args):
-            try:
-                result = data[args]
-                stats[0] += 1  # Hit
-            except KeyError:
-                stats[1] += 1  # Miss
-                if len(data) == maxsize:
-                    keys = data.keys()
-                    for i in xrange(maxsize // 10 or 1):
-                        n = random.randint(0, len(keys) - 1)
-                        k = keys.pop(n)
-                        del data[k]
-                data[args] = user_function(*args)
-                result = data[args]
-            return result
+            with lock:
+                try:
+                    result = data[args]
+                    stats[0] += 1  # Hit
+                except KeyError:
+                    stats[1] += 1  # Miss
+                    if len(data) == maxsize:
+                        keys = data.keys()
+                        for i in xrange(maxsize // 10 or 1):
+                            n = random.randint(0, len(keys) - 1)
+                            k = keys.pop(n)
+                            del data[k]
+                    data[args] = user_function(*args)
+                    result = data[args]
+                return result

         def cache_info():
-            return stats[0], stats[1], maxsize, len(data)
+            with lock:
+                return stats[0], stats[1], maxsize, len(data)

         def cache_clear():
-            data.clear()
+            with lock:
+                data.clear()

         wrapper.cache_info = cache_info
         wrapper.cache_clear = cache_clear
@@ -231,37 +239,41 @@
     def decorating_function(user_function):
         # Cache1, Cache2, Pointer, Hits, Misses
         stats = [{}, {}, 0, 0, 0]
+        lock = Lock()

         @functools.wraps(user_function)
         def wrapper(*args):
-            ptr = stats[2]
-            a = stats[ptr]
-            b = stats[not ptr]
-            key = args
+            with lock:
+                ptr = stats[2]
+                a = stats[ptr]
+                b = stats[not ptr]
+                key = args

-            if key in a:
-                stats[3] += 1  # Hit
-                return a[key]
-            elif key in b:
-                stats[3] += 1  # Hit
-                return b[key]
-            else:
-                stats[4] += 1  # Miss
-                result = user_function(*args)
-                a[key] = result
-                if len(a) >= maxsize:
-                    stats[2] = not ptr
-                    b.clear()
-                return result
+                if key in a:
+                    stats[3] += 1  # Hit
+                    return a[key]
+                elif key in b:
+                    stats[3] += 1  # Hit
+                    return b[key]
+                else:
+                    stats[4] += 1  # Miss
+                    result = user_function(*args)
+                    a[key] = result
+                    if len(a) >= maxsize:
+                        stats[2] = not ptr
+                        b.clear()
+                    return result

         def cache_info():
-            return stats[3], stats[4], maxsize, len(stats[0]) + len(stats[1])
+            with lock:
+                return stats[3], stats[4], maxsize, len(stats[0]) + len(stats[1])

         def cache_clear():
             """Clear the cache and cache statistics"""
-            stats[0].clear()
-            stats[1].clear()
-            stats[3] = stats[4] = 0
+            with lock:
+                stats[0].clear()
+                stats[1].clear()
+                stats[3] = stats[4] = 0

         wrapper.cache_info = cache_info
         wrapper.cache_clear = cache_clear