Intravesical instillation of adriamycin (ADM) or mitomycin C (MMC) was carried out for the purpose of preventing the recurrence of superficial bladder cancers (Ta and T1) after transurethral resection or transurethral coagulation. First, eligible patients were divided into two groups (solitary and multiple) and then they were randomized into the following three groups: (1) ADM group, intravesical instillation with 50 mg ADM dissolved in 100 ml physiological saline; (2) MMC group, intravesical instillation with 30 mg MMC dissolved in 100 ml physiological saline; (3) control, transurethral resection or transurethral coagulation alone. The drugs were given six times by instillation within 2 weeks after TUR or TUC; after 2 weeks the drugs were administered for 2 consecutive days every 4 weeks for 2 years. Of the 134 patients admitted to the study, 103 were evaluable and 31 were eliminated as non-evaluable patients. The cumulative nonrecurrence rates were 73.6% for ADM, 63.4% for MMC, and 22.5% for controls after a follow-up of 24 months. The cumulative non recurrence rates of the ADM and MMC groups were significantly higher than that of the control group. The incidence of side effects was low and the grade of these side effects mild. This instillation therapy with ADM and MMC was considered useful for preventing the recurrence of superficial bladder cancers.