>>You can also do something like, in Lostfocus:
>>
>>if empty(field)
>> =messagebox('Must input value',etc)
>> nodefault
>> object.setfocus (back to field)
>>endif
>
>Hmm...I tried this and it worked. I tried the exact same code before without the 'nodefault' and it didn't work.

this is because when after your call to the set focus the execution returned to the lostfocus which continued to do what it should - lose the focus , when you speicify nodeafult it doesn't execute the default behaviour thus it doesn't loses focus

Arnon