On Got Focus

The Enter and Exit events are more suitable here.  If you use LostFocus, the textbox could still "lose focus" even when the user hasn't tabbed out of it.  That's because when the *form itself* loses focus, such as when you click on another object (report, query, another form, etc.), that makes the textbox "lose focus" too, even if the cursor is still inside the textbox.  Only the Enter and Exit events actually check whether the user has actually tabbed in or out of the textbox.