A retrospective study of 15 patients with primary dysplasia of bladder urothelium was performed in an attempt to clarify some of the clinical and histopathological features. The lesion occurs predominantly in middle aged men who present with irritative bladder symptoms with or without haematuria and show no evidence of bacteriological urinary tract infection. Appearances at cystoscopy are non-specific. Thirteen patients showed no progression to carcinoma in situ after a long period of follow up, but primary dysplasia cannot be regarded as an entirely innocuous condition as the remaining two patients did develop carcinoma in situ. The grade of dysplasia on presentation bears no relation to the final outcome. Regular cytological analysis of urine seems to be the best means of follow up, and more active treatment does not seem to be justified.