Detecting Combo Selection Changed

I need to change the contents of a list box when the selection of a combo box is changed.  The only way I've found to do this is to attach code to the OnLostFocus method of the combo box that checks to see if the selection has changed.  This seems like a clunky way do do it.  Oddly, the Change event doesn't fire when the selection is changed.  Do you know of a way to detect when the combo box selection has changed?

Thanks,

Eric.