diff --git a/lib/memjs/memjs.js b/lib/memjs/memjs.js
index 688456b..1145756 100644
--- a/lib/memjs/memjs.js
+++ b/lib/memjs/memjs.js
@@ -99,7 +99,7 @@ Client.prototype.server = function(key) {
 //
 // Fetches the value at the given key with callback signature:
 //
-//     callback(err, value, flags)
+//     callback(err, value, flags, cas)
 //
 // _value_ and _flags_ are both `Buffer`s.
 // If the key is not found, the callback is invoked
@@ -110,20 +110,20 @@ Client.prototype.get = function(key, callback) {
   var logger = this.options.logger;
   this.perform(key, request, function(err, response) {
     if (err) {
-      callback && callback(err, null, null);
+      callback && callback(err, null, null, null);
       return;
     }
     switch (response.header.status) {
     case  0:
-      callback && callback(null, response.val, response.extras)
+      callback && callback(null, response.val, response.extras, response.header.cas)
       break;
     case 1:
-      callback && callback(null, null, null);
+      callback && callback(null, null, null, null);
       break;
     default:
       var errorMessage = 'MemJS GET: ' + errors[response.header.status];
       logger.log(errorMessage);
-      callback && callback(new Error(errorMessage), null, null);
+      callback && callback(new Error(errorMessage), null, null, null);
     }
   });
 }
@@ -134,12 +134,12 @@ Client.prototype.get = function(key, callback) {
 // in last, after the callback) overrides the default expiration (see
 // `Client.create`). The callback signature is:
 //
-//     callback(err, success)
-Client.prototype.set = function(key, value, callback, expires) {
+//     callback(err, success, cas)
+Client.prototype.set = function(key, value, callback, expires, cas) {
   var extras = Buffer.concat([new Buffer('00000000', 'hex'),
                               makeExpiration(expires || this.options.expires)]);
   this.seq++;
-  var request = makeRequestBuffer(1, key, extras, value, this.seq);
+  var request = makeRequestBuffer(1, key, extras, value, this.seq, cas);
   var logger = this.options.logger;
   this.perform(key, request, function(err, response) {
     if (err) {
@@ -148,7 +148,10 @@ Client.prototype.set = function(key, value, callback, expires) {
     }
     switch (response.header.status) {
     case 0:
-      callback && callback(null, true)
+      callback && callback(null, true, response.header.cas);
+      break;
+    case 2: // Key exists (CAS didn't match)
+      callback && callback(null, false, null);
       break;
     default:
       var errorMessage = 'MemJS SET: ' + errors[response.header.status];
@@ -178,10 +181,10 @@ Client.prototype.add = function(key, value, callback, expires) {
     }
     switch (response.header.status) {
     case 0:
-      callback && callback(null, true)
+      callback && callback(null, true, response.header.cas)
       break;
     case 2:
-      callback && callback(null, false);
+      callback && callback(null, false, null);
       break;
     default:
       var errorMessage = 'MemJS ADD: ' + errors[response.header.status];
@@ -199,10 +202,10 @@ Client.prototype.add = function(key, value, callback, expires) {
 // `Client.create`). The callback signature is:
 //
 //     callback(err, success)
-Client.prototype.replace = function(key, value, callback, expires) {
+Client.prototype.replace = function(key, value, callback, expires, cas) {
   var extras = Buffer.concat([new Buffer('00000000', 'hex'), makeExpiration(expires || this.options.expires)]);
   this.seq++;
-  var request = makeRequestBuffer(3, key, extras, value, this.seq);
+  var request = makeRequestBuffer(3, key, extras, value, this.seq, cas);
   var logger = this.options.logger;
   this.perform(key, request, function(err, response) {
     if (err) {
@@ -211,10 +214,13 @@ Client.prototype.replace = function(key, value, callback, expires) {
     }
     switch (response.header.status) {
     case 0:
-      callback && callback(null, true)
+      callback && callback(null, true, response.header.cas);
       break;
     case 1:
-      callback && callback(null, false);
+      callback && callback(null, false, null);
+      break;
+    case 2:
+      callback && callback(null, false, response.header.cas);
       break;
     default:
       var errorMessage = 'MemJS REPLACE: ' + errors[response.header.status];
diff --git a/lib/memjs/utils.js b/lib/memjs/utils.js
index c1ccfdc..55d93f8 100644
--- a/lib/memjs/utils.js
+++ b/lib/memjs/utils.js
@@ -4,7 +4,7 @@ var bufferify = function(val) {
   return Buffer.isBuffer(val) ? val : new Buffer(val);
 }
 
-exports.makeRequestBuffer = function(opcode, key, extras, value, opaque) {
+exports.makeRequestBuffer = function(opcode, key, extras, value, opaque, cas) {
   key = bufferify(key);
   extras = bufferify(extras);
   value = bufferify(value);
@@ -16,7 +16,8 @@ exports.makeRequestBuffer = function(opcode, key, extras, value, opaque) {
     keyLength: key.length,
     extrasLength: extras.length,
     totalBodyLength: key.length + value.length + extras.length,
-    opaque: opaque
+    opaque: opaque,
+    cas: cas
   };
   header.toBuffer(requestHeader).copy(buf);
   extras.copy(buf, 24)
diff --git a/test/client_test.js b/test/client_test.js
index aaf7585..5899e2c 100644
--- a/test/client_test.js
+++ b/test/client_test.js
@@ -10,14 +10,21 @@ exports.testGetSuccessful = function(beforeExit, assert) {
     assert.equal('hello', request.key);
     n += 1;
     dummyServer.respond(
-      {header: {status: 0, opaque: request.header.opaque},
-        val: 'world', extras: 'flagshere'});
+      {
+        header: {
+          status: 0,
+          opaque: request.header.opaque,
+          cas: new Buffer([0x0a, 0, 0, 0, 0, 0, 0, 0])
+        },
+        val: 'world', extras: 'flagshere'
+      });
   }
 
   var client = new MemJS.Client([dummyServer]);
-  client.get('hello', function(err, val, flags) {
+  client.get('hello', function(err, val, flags, cas) {
     assert.equal('world', val);
     assert.equal('flagshere', flags);
+    assert.deepEqual(new Buffer([0x0a, 0, 0, 0, 0, 0, 0, 0]), cas);
     assert.equal(null, err);
     callbn += 1;
   });
@@ -104,6 +111,34 @@ exports.testSetWithExpiration = function(beforeExit, assert) {
   });
 }
 
+exports.testSetWithCas = function(beforeExit, assert) {
+  var n = 0;
+  var callbn = 0;
+  var dummyServer = new MemJS.Server();
+  dummyServer.write = function(requestBuf) {
+    console.log(requestBuf);
+    request = MemJS.Utils.parseMessage(requestBuf);
+    assert.equal('hello', request.key);
+    assert.equal('world', request.val);
+    assert.deepEqual(new Buffer([0x0a,0,0,0,0,0,0,0]), request.header.cas);
+    n += 1;
+    dummyServer.respond({header: {status: 0, opaque: request.header.opaque}});
+  }
+
+  var client = new MemJS.Client([dummyServer]);
+  client.set('hello', 'world', function(err, val) {
+    assert.equal(null, err);
+    assert.equal(true, val);
+    callbn += 1;
+  }, undefined, new Buffer([0x0a, 0, 0, 0, 0, 0, 0, 0]));
+          
+
+  beforeExit(function() {
+    assert.equal(1, n,  'Ensure set is called');
+    assert.equal(1, callbn,  'Ensure callback is called');
+  });
+}
+
 exports.testSetUnsuccessful = function(beforeExit, assert) {
   var n = 0;
   var callbn = 0;