The frequency of Brunn's nests, cystitis cystica and squamous metaplasia was studied by multiple histologic sections in 100 grossly normal bladders obtained at postmortem examination. Of the 100 bladders 93 per cent had at least 1 of the 3 proliferative changes. Brunn's nests and cystitis cystica were seen in 89 and 60 per cent of the bladders, respectively, most often at the bladder neck and trigone. Non-keratinizing squamous metaplasia of the vaginal type was seen almost exclusively on the trigone and bladder neck in 46 per cent of the premenopausal and postmenopausal women but in only 7 per cent of the men. There was no satisfactory evidence that inflammation was casually associated with these changes. These findings suggest that Brunn's nests, cystitis cystica and squamous metaplasia of the vaginal type in women are normal variants of bladder urothelium and have no precancerous potential.