Access 2003: Combo box based on results from ComboBox

I have a combobox containing a query criteria referring to the results of another combo box.

ie.,

cboMonth: Choose the Month
cboEvent:  Choose the Event based on cboMonth

What do I need to do to update the cboEvent contents if you change cboMonth?
trying requery isn't helping me unless I'm placing it in the wrong event.

Thanks