Idiopathic Urticaria - yeah, that's what they say. The doctors know nothing. First time I had hives was May 2006. Lasted through August. Tried steroids first, benadril, Ranitidine, banadril ointment, Caladryl lotion, was on blant food for a couple of weeks - all in vain. Hives went away in August. No reason - I had changed nothing. Thought I was clear. Till November 2006, when again for no reason - broke up again. Right after Christmas it was gone (I was hfving a bad cold at that time - funny, did it help to cure hives?). Another 2 clear months are gone - and here I go again, March 2007 - another breakout. Suspiciously timely - 2-3 months recess - 2 months with hives. Idiopathic enough.
A friend of mine, who had hives as well, started oral ccontraceptives - and she is free of hives for more tahn a year now. I would think it's a hormone thing, since I got my hives at 40, she is about the same age, and I know some girls who had hives while teenagers.