This is my combobox with Custom validator
 <telerik:RadComboBox  ID="cbSecurityLevel" Skin="Vista" runat="server" ></telerik:RadComboBox>
                                   <asp:CustomValidator ControlToValidate="cbSecurityLevel" Display="Dynamic" CssClass="validator"
                                    OnServerValidate="cbSecurityLevel_ServerValidate" ID="cvSecurityLevel" runat="server"/>

I fill dynamically :
   protected void Page_Load(object sender, EventArgs e)
        {

            if (Page.IsPostBack) return;

            LoadSettings();
            FillComboBox();           
        }

 private void FillComboBox()
        {
    cbSecurityLevel.EmptyMessage = _resources.GetString(cbSecurityLevel.ID, new CultureInfo(_culture));
            cbSecurityLevel.DataSource = _organization.GetSecurityLevels();
            cbSecurityLevel.DataTextField = "Name";
            cbSecurityLevel.DataValueField = "Id";
            cbSecurityLevel.DataBind();
        }


This is Custom validator: (This does not Fire)
 protected void cbSecurityLevel_ServerValidate(object sender, ServerValidateEventArgs e)
        {
            e.IsValid = cbSecurityLevel.SelectedIndex > 0;
        }

Thanks!

this not works :-(

I changed CustomValidor by RequiredFieldValidator and Works!
Thanks