diff --git a/History.md b/History.md
index d016c40ffb..a89be7e134 100644
--- a/History.md
+++ b/History.md
@@ -43,6 +43,9 @@
 
 * Reduced unecessary MongoDB re-polling of live queries.
 
+* Added `Session.setDefault(key, value)` so you can easily provide initial
+  values for session variables that will not be clobbered on hot code push.
+
 * Improved UI for running tinytest package tests in-browser.
 
 * Upgraded UglifyJS2 to version 2.2.5
diff --git a/docs/client/api.html b/docs/client/api.html
index 728448d5ae..2066769ca8 100644
--- a/docs/client/api.html
+++ b/docs/client/api.html
@@ -1163,6 +1163,11 @@ Example:
 
 See [`Meteor.deps`](#meteor_deps) for another example.
 
+{{> api_box setDefault}}
+
+This is useful in initialization code, to avoid re-initializing a session
+variable every time a new version of your app is loaded.
+
 {{> api_box get}}
 
 Example:
diff --git a/docs/client/api.js b/docs/client/api.js
index c442898588..54d55735cf 100644
--- a/docs/client/api.js
+++ b/docs/client/api.js
@@ -1367,6 +1367,21 @@ Template.api.set = {
   ]
 };
 
+Template.api.setDefault = {
+  id: "session_set_default",
+  name: "Session.setDefault(key, value)",
+  locus: "Client",
+  descr: ["Set a variable in the session if it is undefined. Otherwise works exactly the same as [`Session.set`](#session_set)."],
+  args: [
+    {name: "key",
+     type: "String",
+     descr: "The key to set, eg, `selectedItem`"},
+    {name: "value",
+     type: "EJSON-able object or undefined",
+     descr: "The new value for `key`"}
+  ]
+};
+
 Template.api.get = {
   id: "session_get",
   name: "Session.get(key)",
diff --git a/docs/client/docs.js b/docs/client/docs.js
index 613886a013..749ac59b82 100644
--- a/docs/client/docs.js
+++ b/docs/client/docs.js
@@ -161,6 +161,7 @@ var toc = [
 
     "Session", [
       "Session.set",
+      {name: "Session.setDefault", id: "session_set_default"},
       "Session.get",
       "Session.equals"
     ],
diff --git a/examples/todos/client/todos.js b/examples/todos/client/todos.js
index c9b8a1e0b7..dfb38295a5 100644
--- a/examples/todos/client/todos.js
+++ b/examples/todos/client/todos.js
@@ -5,19 +5,19 @@ Lists = new Meteor.Collection("lists");
 Todos = new Meteor.Collection("todos");
 
 // ID of currently selected list
-Session.set('list_id', null);
+Session.setDefault('list_id', null);
 
 // Name of currently selected tag for filtering
-Session.set('tag_filter', null);
+Session.setDefault('tag_filter', null);
 
 // When adding tag to a todo, ID of the todo
-Session.set('editing_addtag', null);
+Session.setDefault('editing_addtag', null);
 
 // When editing a list name, ID of the list
-Session.set('editing_listname', null);
+Session.setDefault('editing_listname', null);
 
 // When editing todo text, ID of the todo
-Session.set('editing_itemname', null);
+Session.setDefault('editing_itemname', null);
 
 // Subscribe to 'lists' collection on startup.
 // Select a list once data has arrived.
@@ -66,6 +66,7 @@ var okCancelEvents = function (selector, callbacks) {
           cancel.call(this, evt);
       }
     };
+
   return events;
 };
 
@@ -304,8 +305,11 @@ var TodosRouter = Backbone.Router.extend({
     ":list_id": "main"
   },
   main: function (list_id) {
-    Session.set("list_id", list_id);
-    Session.set("tag_filter", null);
+    var oldList = Session.get("list_id");
+    if (oldList !== list_id) {
+      Session.set("list_id", list_id);
+      Session.set("tag_filter", null);
+    }
   },
   setList: function (list_id) {
     this.navigate(list_id, true);
diff --git a/packages/session/session.js b/packages/session/session.js
index a8aade94d3..226668cec0 100644
--- a/packages/session/session.js
+++ b/packages/session/session.js
@@ -40,6 +40,16 @@
       }
     },
 
+    setDefault: function (key, value) {
+      var self = this;
+      // for now, explicitly check for undefined, since there is no
+      // Session.clear().  Later we might have a Session.clear(), in which case
+      // we should check if it has the key.
+      if (self.keys[key] === undefined) {
+        self.set(key, value);
+      }
+    },
+
     get: function (key) {
       var self = this;
       self._ensureKey(key);
diff --git a/packages/session/session_tests.js b/packages/session/session_tests.js
index 2cee082469..97b00d9762 100644
--- a/packages/session/session_tests.js
+++ b/packages/session/session_tests.js
@@ -1,5 +1,21 @@
 (function () {
 
+  Tinytest.add('session - setDefault', function (test) {
+    Session.setDefault('def', "argyle");
+    test.equal(Session.get('def'), "argyle");
+    Session.set('def', "noodle");
+    test.equal(Session.get('def'), "noodle");
+    Session.set('nondef', "potato");
+    test.equal(Session.get('nondef'), "potato");
+    Session.setDefault('nondef', "eggs");
+    test.equal(Session.get('nondef'), "potato");
+    // This is so the test passes the next time, after hot code push.  I know it
+    // doesn't return it to the completely untouched state, but we don't have
+    // Session.clear() yet.  When we do, this should be that.
+    delete Session.keys['def'];
+    delete Session.keys['nondef'];
+  });
+
   Tinytest.add('session - get/set/equals types', function (test) {
     test.equal(Session.get('u'), undefined);
     test.isTrue(Session.equals('u', undefined));