Hi Mel:

>I am pretty sure I would use the IsNewAllowedRule() event for this, but can't figure out the code.

Yes, you could use this method. Also, back to our old friend IsEnabled() (which calls IsNewAllowedRule in its default code for the "New" method), you could say something like :-

Function Form.IsEnabled
... 

Case m.cMethod == 'New'   && Dealing with the New method.
If m.cWorkarea == 'Childtable'
   If Thisform.IsNew('ParentTable')
      *-- We have an un-committed new parent record so disallow new for "Childtable".
      Return .F.
   Endif
Endif
Return .T.

Obviously, this is the sort of code you could put in the IsNewAllowedRule method.

HTH