Ah... you're using a master page. You need to either use the 'complex' id or

var ctl = document.getElementById("<%= this.txtControl.ClientID %>");

to embed the value correctly.

+++ Rick ---

>>Script like this is usually placed in 'startup code' which is a script block just before the form tag.
>>
>>That works in most cases or alternately you need a real startup handler like hooking < body onload="" > or using a javascript library like jQuery to reliably tell when the document's properly loaded.
>>
>>Overkill in your situation - you can just use the startup script block on the bottom of the page and that will work fine.
>>
>>+++ Rick ---
>
>My problem is what tblProInfo is the name from the Server. In the HTML code I see
>ct100_ContentPlaceHolder1_tblProInfo instead.
>
>Can I pass the ClientID to the JavaScript from OnCLientClick. Right now I created a little function HideElement
>
>

><script type="text/javascript">
>function HideElement(what)
>  {
>  var x = document.getElementById(what);
>  //alert(x);
>  if (x) {
>     x.style.display = "none"; }
>  }
></script>

>
>And I call it from ASPX OnCLientClick "HideElement('tblProInfo')". I need to pass ClientID here instead.
>
>Thanks a lot in advance.