http://www.utteraccess.com/wiki/index.php/Cascading_Combo_Boxes
http://www.fmsinc.com/microsoftaccess/Forms/combo-boxes/cascading.html
Private Sub ApplyFilter()
Dim strFilter As String
strFilter = ""
' see if there is data in combo box cboA, if so add it to the filter
If Me!cboA & vbNullStr <>...