A randomized controlled prospective evaluation of intravesical and percutaneous bacillus Calmette-Guerin immunotherapy was done in 57 patients with transitional cell carcinoma of the bladder. In addition, 9 patients at high risk for tumor recurrence were treated with bacillus Calmette-Guerin produced a self-limited cystitis and 1 complication (hydronephrosis) of immunotherapy was observed. Of the 57 randomized patients 54 were followed for 3 to 30 months. Tumor recurrence was documented in 13 of 26 controls (50 per cent) and only 6 of 28 patients (21 per cent) treated with bacillus Calmette-Guerin (p equals 0.027, chi-square). The interval free of disease was prolonged significantly with bacillus Calmette-Guerin treatment (p equals 0.014, generalized Wilcoxon test). Importantly, a simple purified protein derivative skin test distinguished those patients who responded to bacillus Calmette-Guerin immunotherapy from those who did not. Only 1 of 17 treated patients (6 per cent) whose purified protein derivative test converted from negative to positive had tumor recurrence compared to 5 recurrences (38 per cent) among the 13 patients whose test remained negative or had been positive before treatment (p equals 0.022, chi-square). Bacillus Calmette-Guerin was given to 10 patients with stage B transitional cell carcinoma who were not candidates for cystectomy and 7 are free of disease. Of 5 patients with carcinoma in situ 3 remain free of tumor after bacillus Calmette-Guerin treatment and 5 of 6 who had multiple recurrences after intravesical chemotherapy responded favorably to bacillus Calmette-Guerin immunotherapy.