>I want multiple forms in a formset, form1 will control which of the other forms (form2-formn) becomes active.
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>Since this is double-entry medical data, the subsequent forms must be modal. I have tried setting the WindowType property, and tried using Show(1), and I can still return to form1. I can load forms 2 and 3 and go between the two. How do I make them really, really modal?

Write some code to disable 'switching' controls (some buttons?) when you go to 'really modal' form (e.g. in Init event) and enable them in Destroy event.