I recently came across the AsyncFileUpload control in the latest (3.0.40412) release of the ASP.Net Ajax Control Toolkit. There appears to be an issue when using it in a hidden control that is later revealed, such as a <div> tag with visible=false. Example: Page code - <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="act" %> . . . <act:ToolkitScriptManager runat="server" ID="ScriptManager1" /> <asp:UpdatePanel runat="server" ID="upnlFileUpload"> <ContentTemplate> <asp:Button runat="server" ID="btnShowUpload" Text="Show Upload" /> <div runat="server" id="divUpload" visible="false"> <act:AsyncFileUpload runat="server" id="ctlFileUpload" /> </div> </ContentTemplate> </asp:UpdatePanel> Server-side Code - Protected Sub ctlFileUpload_UploadedComplete(ByVal sender As Object, ByVal e As AjaxControlToolkit.AsyncFileUploadEventArgs) Handles ctlFileUpload.UploadedComplete End Sub Protected Sub btnShowUpload_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnShowUpload.Click divUpload.Visible = True End Sub I have a breakpoint on the UploadedComplete event but it never fires. However, if you take the AsyncFileUpload control out of the <div>, making it visible at initial page render, the control works as expected. So, is this a bug within the AsynchUploadControl, or am I not grasping a fundamental concept (which happens regularly)?