Fifty-one patients with confirmed bladder cancer have enrolled in a prospective evaluation of BCG immunotherapy. Following resection of existing tumors, patients were stratified according to tumor grade and number of previous recurrences and randomly assigned to control or BCG treatment groups. Immunotherapy consisted of six weekly administrations of Pasteur strain BCG using 120 mg intravesically and 5 mg percutaneously. Immunotherapy side effects were minimal and no patient required postponement of BCG treatments. Eleven control (46%) compared with five (22%) BCG-treated patients had tumor recurrence (P = 0.078, chi 2). Prolongation of the disease-free interval with BCG treatment was significantly at the P = 0.016 level by Wilcoxon analysis. Four control and two BCG-treated patients had multiple recurrences. Comparing total episodes of recurrence, nineteen of 79 (24%) control and eight of 85 (7%) BCG group cystoscopic examinations revealed tumor (P = 0.006, chi 2). Immunologic correlates of response to immunotherapy were not statistically significant since only five BCG-treated patients had tumor recurrence. However, four of these five patients evidenced impaired LIF response to PPD at the time of tumor recurrence, and impairment of skin test reactivity and BCG humoral antibody response were more commonly seen in this subgroup of patients.