Granulomatous prostatitis is a recognized complication of intravesical bacillus Calmette-Guerin immunotherapy of superficial bladder cancer. Of 32 patients receiving such therapy 13 underwent prostatic core biopsy and/or fine needle aspiration for clinical indications. Prostatic induration or nodularity developed in 12 patients and 1 underwent biopsy for staging of known prostatic carcinoma. Granulomatous prostatitis was found in 100 per cent of those patients who underwent biopsy or aspiration, indicating that the incidence of this finding is at least 41 per cent following bacillus Calmette-Guerin immunotherapy. Acid-fast bacilli were demonstrated within the prostate of 3 patients with granulomatous prostatitis. The mean interval between the initiation of therapy and diagnosis of granulomatous prostatitis was 11.5 months. Bacillus Calmette-Guerin related granulomas of the prostate may be differentiated histologically from nonspecific granulomatous prostatitis, allergic prostatitis and postoperative granulomas. The clinical implications of these findings are discussed.