hide ListBox control vertical scrollbar

i have a listbox but i want the vertical scroll bar show in the listbox.
it doesn't seem in ie but chrome seems..
<asp:ListBox ID="urun" runat="server"  Height="100" SelectionMode="Multiple" CssClass="listbox" Enabled="false">
<asp:ListItem Text="item1" Value="" >item1</asp:ListItem>
<asp:ListItem Text="item1" Value="" >item1</asp:ListItem>
<asp:ListItem Text="item1" Value="" >item1</asp:ListItem>
<asp:ListItem Text="item1" Value="" >item1</asp:ListItem>
</asp:ListBox>
...