A consecutive series of 500 primary bladder tumours from a single clinic is presented, with distribution of the tumours according to T category and histologic type and grade. Mucosal biopsies were obtained from pre-selected sites at initial cystoscopy or initial transurethral resection of the tumour in 396 cases. In 54% of the patients with grade III tumour there was concomitant urothelial atypia, either carcinoma in situ (urothelial atypia grade III, 30%) or urothelial atypia grade II (24%). In 30% of the patients with invasive grade II bladder tumour and in 14% of those with noninvasive grade II tumour there was concomitant urothelial atypia, mostly grade II. Since concomitant urothelial atypia predicts new tumour growth after successful transurethral surgery or radiotherapy, mucosal biopsies should be performed at preselected sites during initial cystoscopy or transurethral tumour resection in order to identify high-risk patients.