or

<telerik:RadTreeView Runat="Server" ID="RadTreeView1" Skin="Vista" OnNodeExpand = "Rad_NodeExpand" >
</telerik:RadTreeView>
Code:
protected void Rad_NodeExpand(object sender, RadTreeNodeEventArgs e) { if (e.Node.Nodes.Count == 0) { switch (e.Node.Level) { case 0: FillTerritories(e.Node); break; case 1: FillOrganizations(e.Node); break; case 2: FillOffices(e.Node); break; } } }<telerik:radajaxpanel id="RadAjaxPanel1" runat="server" clientevents-onrequeststart="conditionalPostback"> <telerik:RadAjaxManager ID="RadAjaxManager1" runat="server"> <AjaxSettings> <telerik:AjaxSetting AjaxControlID="RadGrid1"> <UpdatedControls> <telerik:AjaxUpdatedControl ControlID="RadGrid1" LoadingPanelID="RadAjaxLoadingPanel1" /> <telerik:AjaxUpdatedControl ControlID="RadWindowManager1" /> </UpdatedControls> </telerik:AjaxSetting> </AjaxSettings> </telerik:RadAjaxManager> <telerik:RadAjaxLoadingPanel ID="RadAjaxLoadingPanel1" runat="server" BackColor="#CCCCCC" Transparency="30" HorizontalAlign="Center" BorderStyle="NotSet"> <div style="margin: 0 auto; padding-top: 50px;"><img src="../images/ajax-loader.gif" alt="Loading..." border="0" /></div> </telerik:RadAjaxLoadingPanel> <telerik:RadGrid ID="RadGrid1" runat="server" AllowAutomaticDeletes="true" AllowAutomaticUpdates="true" AllowPaging="true" AllowFilteringByColumn="true" AllowSorting="true" OnDeleteCommand="DeleteUser" OnUpdateCommand="UpdateUser" PageSize="20" OnEditCommand="EditUser" OnNeedDataSource="RadGrid1_NeedDataSource" OnInsertCommand="InsertUser" AllowAutomaticInserts="true" Width="100%" Skin="Sunset"> <PagerStyle Mode="NextPrevAndNumeric" /> <MasterTableView DataKeyNames="ID" AutoGenerateColumns="false" CommandItemDisplay="Top"><div id="ctl00_ContentPlaceHolder1_ctl00_ContentPlaceHolder1_RadAjaxPanel1Panel" style="display: block; "> <div id="ctl00_ContentPlaceHolder1_RadAjaxPanel1"> <!-- 2010.3.1317.40 --> <div id="ContentPlaceHolder1_RadAjaxManager1SU" style="display: block; "> <span id="ctl00_ContentPlaceHolder1_RadAjaxManager1" style="display:none;"></span> </div> <div id="ContentPlaceHolder1_RadAjaxLoadingPanel1" style="display:none;background-color:#CCCCCC;text-align:center;"> <div style="margin: 0 auto; padding-top: 50px;"><img src="../assets/ajax-loader.gif" alt="Loading..." border="0"></div> </div> <div id="ctl00_ContentPlaceHolder1_ctl00_ContentPlaceHolder1_RadGrid1Panel" style="display: block; "> <div id="ctl00_ContentPlaceHolder1_RadGrid1" class="RadGrid RadGrid_Default" tabindex="0"> <table cellspacing="0" class="rgMasterTable" id="ctl00_ContentPlaceHolder1_RadGrid1_ctl00" style="width:100%;table-layout:auto;empty-cells:show;"><div id="ctl00_ContentPlaceHolder1_ctl00_ContentPlaceHolder1_RadGrid1Panel" style="display: block; "><telerik:RadPanelBar runat="server" ID="LeftMenu" Width="200" ExpandMode="SingleExpandedItem" AllowCollapseAllItems="true" CollapseAnimation-Type="None" CollapseAnimation-Duration="0" ExpandAnimation-Duration="0" ExpandAnimation-Type="None" PersistStateInCookie="false" EnableEmbeddedBaseStylesheet="true" EnableEmbeddedSkins="false" EnableViewState="true" CollapseDelay="0" ExpandDelay="0" > </telerik:RadPanelBar>