A prospective study was done on 52 patients with bladder cancer to determine the incidence of atypia, carcinoma in situ and carcinoma in selected site biopsies from normal-appearing mucosa at the time of initial and subsequent endoscopy. Biopsies at initial endoscopy revealed abnormalities in 33 per cent of the patients, while 77 per cent of 43 patients undergoing 3-month selected site biopsies had at least 1 abnormal biopsy during a 1-year period.