RETURN .F. in valid

>Hi! I have a form indexed on Estimate Number.
>
>I want to force the user to enter an estimate number, if they forget, as soon as the field loses focus a messagebox appears giving the user the chance to retry or cancel.
>
>Cancel leaves the add, no problem there.
>
>I want the retry to set the focus back to the estimate number? Since my code is in the lost focus of my field and I have a thisform.txtNEstimateNo.setfocus() in the method, the focus still moves to the next field?
>
>How can I stop this behaviour?
>
>MTIA