A total of 208 patients with superficial transitional-cell carcinoma of the bladder (STCC) after transurethral resection were treated with 30 mg intravesical instillations of mitomycin C (MMC) weekly for 8 weeks, followed by monthly maintenance doses for 12 months. All patients were controlled with a urinary cytological examination every 2 months and with cystoscopy every 3 months. Mean follow-up was 47.8 and 49.3 months in the prophylaxis and control groups, respectively. The incidence of tumor recurrences at the 12th and 48th months was 29 and 44%, respectively, in the MMC group and 45 and 58%, respectively, in the control group. Progression evaluated by grade and stage was significantly higher in the control group. These data indicate that MMC appears to be effective in the prophylaxis of STCC, but the possibility of long-term relapse suggests maintenance of a longer therapy.