A phase II study was performed to assess the role of bacillus Calmette-Guerin as a prophylaxis against recurrent stages O and A bladder tumors, and in the treatment of existing superficial bladder tumors and carcinoma in situ. Tice strain bacillus Calmette-Guerin (1 vial, 2 to 8 times 108 organisms in 60 cc saline) was instilled intravesically without cutaneous inoculation. Instillations were given weekly for 6 weeks and then monthly or until recurrence in 22 patients with a history of recurrent tumors, while 22 with existing stages O and A transitional cell carcinoma, and 19 with carcinoma in situ were treated weekly for 8 weeks and then monthly for 12 months or until failure. Complications included cystitis in 88 per cent of the patients (severe in 20 per cent), fever in 15 per cent, a flu-like syndrome in 13 per cent, edema and pruritis in 1.5 per cent, and ureteral stenosis in 1.5 per cent. Twelve patients (19 per cent) did not complete the study owing to toxicity. Of the patients in the prophylaxis group 67 per cent have had no tumor recurrence 10 to 26 months (mean 15 months) after therapy. Of the patients with existing tumors 36 per cent had complete regression following bacillus Calmette-Guerin therapy and 23 per cent had a partial response. Among the patients with carcinoma in situ 13 (68 per cent) had reversal to normal urothelium and 3 (16 per cent) had marked improvement. None of the patients had recurrence at 11 to 20 months. Intravesical Tice strain bacillus Calmette-Guerin is effective as a prophylaxis against recurrent superficial bladder tumors and in the treatment of carcinoma in situ.