I have a doctor who dictates "Raynaud phenomena," but reference books (and some medical sites on the Internet) list it as "Raynaud phenomenon." Does someone know which one would be correct most, if not all, of the time?

Well, the way I understand it, phenomena is the plural of phenomenon. So, the correct spelling would be Raynaud phenomenon.

You could also check Dorlands for Raynaud phenomenon , here, as well.

Hope that helps.

That's exactly my thought, too. Do you know of any disease or syndrome that uses phenomena?

Off the top of my head, I would say no.

I checked in my Dorland's Medical Speller and phenomena is not even listed (except to show that it is the plural of phenomenon).