The issue can be workarounded by calling preventDefault of the mousedown event when ComboBox item is clicked: <telerik:RadScriptManager runat="server" /> <script> var onDocumentClick = Telerik.Web.UI.RadComboBox.prototype._onDocumentClick; Telerik.Web.UI.RadComboBox.prototype._onDocumentClick = function (e) { if (!e) e = event; var target = e.target || e.srcElement; while (target.nodeType !== 9) { if (!target.parentNode || target == this.get_element() || target == this.get_dropDownElement()) { e.preventDefault(); return; } target = target.parentNode; } onDocumentClick.call(this, e); }; </script> <telerik:RadComboBox ID="RadComboBox1" runat="server" RenderMode="Lightweight" OnClientBlur="OnClientBlur" CheckBoxes="true"> <Items> <telerik:RadComboBoxItem Text="Item1" Value="1" /> <telerik:RadComboBoxItem Text="Item2" Value="2" /> <telerik:RadComboBoxItem Text="Item3" Value="3" /> <telerik:RadComboBoxItem Text="Item4" Value="4" /> </Items> </telerik:RadComboBox> <script> function OnClientBlur(sender, args) { console.log("Blur event"); } </script>