Q: Base window turn up with each MessageBox

Hi,
I have a dialog-based application, that starts with one window, from which the user can open other - modal - dialogs. From these dialogs again further modal dialogs can be opened. When such a dialog is opened, the first window (the "base" window, or "grand-father") suddenly becomes visible with the new dialog and remains visible, until the "grand-child" is closed. this occures also with MsgBox'es.
I guess, there is one parameter I have to set accordingly to avoid this behaviour.
Could please someone help?
Thanks
Robert Maier