Intravesical bacillus Calmette-Guerin is effective therapy for multifocal carcinoma in situ of the bladder. The duration of this favorable response and its effect on disease progression are the subject of this report. Between March 1978 and July 1981, 47 patients with diffuse, often symptomatic, carcinoma in situ were treated with intravesical bacillus Calmette-Guerin and followed every 3 to 4 months with cystoendoscopy, biopsy and urine cytology for 3 to 6 years. All patients had had prior or concurrent superficial papillary tumors controlled initially by transurethral resection and fulguration 2 to 3 weeks before bacillus Calmette-Guerin treatment. Of the 47 patients 23 were entered into a randomized study, and received intravesical and percutaneous bacillus Calmette-Guerin. Another 24 patients with carcinoma in situ were treated with intravesical bacillus Calmette-Guerin alone. Bacillus Calmette-Guerin (Pasteur strain) was given intravesically (120 mg. in 50 ml. saline) weekly for 6 weeks. Of the 47 patients 32 (68 per cent) are free of disease (negative urine cytology, cystoendoscopy and biopsy): 15 (65 per cent) after combined bacillus Calmette-Guerin for a median duration of 51 months (range 37 to 75 months) and 17 (71 per cent) after intravesical bacillus Calmette-Guerin alone for a median of 45 months (range 36 to 53 months). Of the 23 patients in the randomized study 4 (17 per cent) have required cystectomy for local progression of disease compared to 17 of 26 controls (65 per cent) who were randomized to transurethral resection and fulguration alone. Cystectomy was performed 3 to 27 months after bacillus Calmette-Guerin treatment and in 3 patients tumor was localized to the prostate gland (no tumor found within the bladder). These data indicate that intravesical bacillus Calmette-Guerin is capable of producing long-term remissions of carcinoma in situ in high risk patients and may prevent or delay progression of disease necessitating cystectomy.