This study presents the preliminary results of a randomized prospective two-arm study in which bacillus Calmette-Guérin (BCG) RIVM, a Dutch BCG preparation, is compared with mitomycin C (MMC) in patients with primary or recurrent superficial bladder tumors, including carcinoma in situ (CIS). Therapeutic regimens were as follows: after complete transurethral resection of all visible tumors, BCG RIVM (1 x 10(9) bacilli in 50 mL saline) was instilled once a week for six consecutive weeks, and mitomycin C (30 mg in 50 mL saline) was administered once a week for one month (weeks 1 to 4) and thereafter once a month for a total of six months. Reported are the incidence of side effects in 165 patients and the recurrence rate of tumors in 308 patients after a follow-up period of twelve months. Drug-induced, or chemical cystitis was observed in 13 (16.7%) of 78 BCG-treated patients and in 12 (13.8%) of 87 MMC-treated patients. In the same groups bacterial cystitis occurred in 17 (21.8%) patients and in 16 (18.4%) patients, respectively. In the BCG-treated group (N = 148), 44 (29.8%) had recurrent tumors, while in the MMC-treated group (N = 160), 40 (25.0%) had a recurrence. The recurrence rate for BCG-treated patients was 0.33; the recurrence rate for MMC-treated patients was 0.29 (P = 0.560, not significant). These preliminary data demonstrated no statistically significant difference between the two arms with regard to toxicity and recurrence of tumors.