This is what MS say about the Red X for dialogs: “The Close button on the title bar should have the same effect as the Cancel or Close button within the dialog box. Never give it the same effect as OK.”

So to respond to Dick’s comment, I would say that if your Cancel button prompts the user in the case of “IsDirty = true” then the red X should have the same effect. That is, if your form has a Cancel button, the red X should cancel and if it has a Close (or no button at all) it should Close. It sounds, however like your Cancel button does NOT prompt in which case to me it seems like you have a different behavior for the red X almost as if you want to ensure your users really meant to exit via that route. I guess that issue is more subjective. We don’t typically do this (red X cancels with no prompt if there is a Cancel button on the dialog), but I wonder if we are giving our users too much credit for knowing that red X will abandon changes for those use cases.

I don’t *always* mimic the Cancel button, but that’s what usually makes the most sense in my usage. The user decides not to graph that data, or create that pivot table, or export that report, and Cancel fits best of all. If unsaved data is an issue, I may be checking in the Cancel button code anyway. Or my buttons will be labeled “Save”, “Save and Exit”, and “Exit Without Saving”. Best practices include using good, descriptive control labels.

In my experience, it seems to me that the red X is for the user who says “let me the hell out of here!”, so I generally just use Cancel.