How to populate list box using ajax

What I want to do is to populate a listbox based on a selection from another listbox using ajax. In short, you click and make a selection in a listbox, pass the value via an ajax call and return a recordset from a mysql query and populate a second listbox. The returned data would contain both value and text to load the listbox. I've tried to find code on the net, but can't seem to find what I need. If someone has some code or reference to code I'd appreciate the help.

Tom