diff --git a/packages/livedata/livedata_common.js b/packages/livedata/livedata_common.js
index 87320ef5d2..f03f1ce0f7 100644
--- a/packages/livedata/livedata_common.js
+++ b/packages/livedata/livedata_common.js
@@ -1,7 +1,6 @@
 // XXX namespacing
 
-Meteor._MethodInvocation = function (isSimulation, userId,
-                                     globallySetUserId, unblock) {
+Meteor._MethodInvocation = function (options) {
   var self = this;
 
   // true if we're running not the actual method, but a stub (that is,
@@ -11,22 +10,28 @@ Meteor._MethodInvocation = function (isSimulation, userId,
   // purposes). not currently true except in a client such as a browser,
   // since there's usually no point in running stubs unless you have a
   // zero-latency connection to the user.
-  this.isSimulation = isSimulation;
+  this.isSimulation = options.isSimulation;
 
   // XXX Backwards compatibility only. Remove this before 1.0.
-  this.is_simulation = isSimulation;
+  this.is_simulation = this.isSimulation;
 
   // call this function to allow other method invocations (from the
   // same client) to continue running without waiting for this one to
   // complete.
-  this.unblock = unblock || function () {};
+  this.unblock = options.unblock || function () {};
 
   // current user id
-  this._userId = userId;
+  this._userId = options.userId;
 
   // sets current user id in all appropriate server contexts and
   // reruns subscriptions
-  this._setUserId = globallySetUserId || function () {};
+  this._setUserId = options.setUserId || function () {};
+
+  // Scratch data scoped to this connection (livedata_connection on the
+  // client, livedata_session on the server). This is only used
+  // internally, but we should have real and documented API for this
+  // sort of thing someday.
+  this._sessionData = options.sessionData;
 };
 
 _.extend(Meteor._MethodInvocation.prototype, {
diff --git a/packages/livedata/livedata_connection.js b/packages/livedata/livedata_connection.js
index 58fde708b2..3acecc30b7 100644
--- a/packages/livedata/livedata_connection.js
+++ b/packages/livedata/livedata_connection.js
@@ -75,6 +75,10 @@ Meteor._LivedataConnection = function (url, options) {
   // yet ready.
   self.sub_ready_callbacks = {};
 
+  // Per-connection scratch area. This is only used internally, but we
+  // should have real and documented API for this sort of thing someday.
+  self.sessionData = {};
+
   // just for testing
   self.quiesce_callbacks = [];
 
@@ -319,8 +323,11 @@ _.extend(Meteor._LivedataConnection.prototype, {
         var setUserId = function(userId) {
           self.setUserId(userId);
         };
-        var invocation = new Meteor._MethodInvocation(
-          true /* isSimulation */, self.userId(), setUserId);
+        var invocation = new Meteor._MethodInvocation({
+          isSimulation: true,
+          userId: self.userId(), setUserId: setUserId,
+          sessionData: self.sessionData
+        });
         try {
           var ret = Meteor._CurrentInvocation.withValue(invocation,function () {
             return stub.apply(invocation, args);
diff --git a/packages/livedata/livedata_server.js b/packages/livedata/livedata_server.js
index 2ef1423ef7..84038f4ede 100644
--- a/packages/livedata/livedata_server.js
+++ b/packages/livedata/livedata_server.js
@@ -38,6 +38,10 @@ Meteor._LivedataSession = function (server) {
   self.dontFlush = false;
 
   self.userId = null;
+
+  // Per-connection scratch area. This is only used internally, but we
+  // should have real and documented API for this sort of thing someday.
+  self.sessionData = {};
 };
 
 _.extend(Meteor._LivedataSession.prototype, {
@@ -281,8 +285,12 @@ _.extend(Meteor._LivedataSession.prototype, {
         self._setUserId(userId);
       };
 
-      var invocation = new Meteor._MethodInvocation(
-        false /* isSimulation */, self.userId, setUserId, unblock);
+      var invocation = new Meteor._MethodInvocation({
+        isSimulation: false,
+        userId: self.userId, setUserId: setUserId,
+        unblock: unblock,
+        sessionData: self.sessionData
+      });
       try {
         var ret =
           Meteor._CurrentWriteFence.withValue(fence, function () {
@@ -437,6 +445,12 @@ Meteor._LivedataSubscription = function (session, sub_id, priority) {
   // LivedataSession
   this.session = session;
 
+  // Give access to sessionData in subscriptions as well as
+  // methods. This is not currently used, but is included for
+  // consistency. We should have real and documented API for this sort
+  // of thing someday.
+  this._sessionData = session.sessionData;
+
   // my subscription ID (generated by client, null for universal subs).
   this.sub_id = sub_id;
 
@@ -873,8 +887,11 @@ _.extend(Meteor._LivedataServer.prototype, {
         };
       }
 
-      var invocation = new Meteor._MethodInvocation(
-        false /* isSimulation */, userId, setUserId);
+      var invocation = new Meteor._MethodInvocation({
+        isSimulation: false,
+        userId: userId, setUserId: setUserId,
+        sessionData: self.sessionData
+      });
       try {
         var ret = Meteor._CurrentInvocation.withValue(invocation, function () {
           return handler.apply(invocation, args);