A total of 130 primary cases with superficial bladder cancer were entered in the prospective randomized group study. The prophylactic treatments compared consisted in intravesical instillation of adriamycin (20 mg/40 ml or 30 mg/30 ml), mitomycin C (20 mg/40 ml) or thio-TEPA (30 mg/30 ml), and noninstillation treatments with etretinate or tegafur; control patients were also studied. All agents were administered for 2 years. Recurrences were significantly suppressed in the instillation groups compared with control and non-instillation groups. Significant suppression of recurrence was observed in stage 1 or grade 2 disease treated with prophylactic instillation administered over the first 24 months of a 48-month observation period. These results may indicate the clinical usefulness of prophylactic instillation, but the long-term effect of intravesical instillation is still uncertain. A long-term follow-up study is therefore necessary.