>Yes the Listbox object is in a form and it is already modal

Are you saying you add the listbox to the form after it's already displayed and then want to wait for a selection? That's a pretty strange UI.

Do you need other inputs from the form in order to populate the listbox? If so, maybe put it in a separate form that you pop up over the current form? If not, just populate it on the way in.

Tamar