The DropDownTree pop-up is not positioned properly when the <form> is wrapped inside a relatively positioned element with fixed width. The issue can be reproduced with a similar configuration: <div style="width: 700px; position: relative; left:300px"> <form id="form1" runat="server"> <telerik:RadScriptManager runat="server"></telerik:RadScriptManager> <div class="position"> <telerik:RadDropDownTree runat="server" ID="ddt1" CssClass="dropDownTree"></telerik:RadDropDownTree> </div> </form> </div>