Thirty-seven patients were enrolled in a randomized prospective study to compare standard surgical therapy for superficial bladder cancer to standard therapy plus bacillus Calmette-Guerin (BCG). Side effects of BCG have been tolerated well and include dysuria in 95 per cent of the patients, urinary frequency in 83 per cent, hematuria in 39 per cent, fever in 22 per cent and nausea in 22 per cent. Of 19 control patients 8 (42 per cent) had recurrent tumors in the followup period, compared to 3 of 18 patients (17 per cent) treated with BCG. One patient treated wih BCG had 2 recurrences, yielding a recurrence rate of 22 per cent in the group receiving BCG compared to 42 per cent in controls. When the incidence of recurrent tumors in matched intervals before and after entry into the protocol is compared, no change in the rate of tumor recurrence (p equals 0.726 chi-square) occurred in controls, whereas tumor recurrences were reduced significantly in the group treated with BCG (p equals 0.010 chi-square). The reduction in tumor recurrence in patients treated with BCG compared to controls is statistically significant (p equals 0.029 chi-square). Of 4 patients who presented with new bladder tumors remain free of tumor after BCG therapy, while 2 of 5 comparable control patients developed recurrent tumors. Intravesical and percutaneous BCG immunotherapy appears to decrease the rate of tumor recurrence in patients followed for 1 year.