The event is always the SelectedIndexChanged event. If you select an item, you can see the new selected index. If you are interested in the previous selected index, then the thing to do is to store the current selected index (which probably starts out as -1) in some form level variable, then on the next SelectedIndexChanged event you have the current SelectedIndex, and the last selected index will be held in that form level variable. Just be sure to update that variable in the SelectedIndexChanged event handler.