Frank,

As it turns out this is an issue with ASP.NET. The code that retrieves the value from the ListBox really should work, but doesn't when you include the page inside a master page. To get this to work properly, you just need to add one line of code to lstUsers_SelectedIndexChanged():

try
{
    this.lstUsers.SelectedValue = Request.Form[lstUsers.UniqueID];
    UserPk = this.lstUsers.SelectedValue;
}

Obviously, you can change this to one line of code and simply retrieve the value using Request.Form[] and then store the value in the UserPK variable.

Best Regards,