A randomized study was done on 49 patients with a history of recurrent superficial transitional cell carcinoma of the bladder to determine if the frequency of tumor recurrence could be lessened. The patients were treated with 6 weekly instillations of Tice strain bacillus Calmette-Guerin or thio-tepa, followed by instillations every 2 weeks for 3 months and every month until a recurrence developed or the patients had been treated for 2 years. Twelve additional patients who had previously failed on thio-tepa were placed on the bacillus Calmette-Guerin protocol. There have been no recurrences in the bacillus Calmette-Guerin group, while 9 patients (40 per cent) in the thio-tepa group suffered recurrences. All of the patients treated with bacillus Calmette-Guerin had symptoms of bladder irritability after each instillation. The same type of irritability occurred in 7 patients (25 per cent) in the thio-tepa group. Severe complications occurred in 11 patients treated with bacillus Calmette-Guerin (28 per cent), 4 of whom required hospitalization. Another 12 patients with visible transitional cell carcinoma were treated with weekly instillations of bacillus Calmette-Guerin. Of these patients 6 had total resolution after 18 weekly instillations and 2 had no detectable tumor after 24 weeks of therapy. Two patients were free of tumor after 6 instillations. Of 7 patients with carcinoma in situ treated with weekly instillations of bacillus Calmette-Guerin 5 had complete resolution of the cellular atypia after 18 weeks of therapy. These studies confirm and extend those of others by demonstrating that bladder instillations of bacillus Calmette-Guerin are effective in reducing the recurrence rate in patients with superficial bladder tumor and can eliminate existing tumor in some patients. Symptoms of bladder irritability occur to a varying extent in every patient, and severe systemic toxicity occurred in 28 per cent of the patients.