PURPOSE: A prospective study was performed to compare the efficacy and safety of intravesical mitomycin C (MMC) instillation for the prophylaxis of bladder cancer at different concentrations (30 mg or 40 mg). MATERIALS AND METHODS: Ninety-seven patients that received complete transurethral resection for superficial bladder cancer were divided into two-randomized groups. One group (n=53) received 30 mg and the other group (n=44) received 40 mg dose of MMC weekly for 8 weeks, which was followed monthly for 10 months as maintenance therapy. The recurrence rates and side effects in both groups were recorded. The mean follow-up period was 32.4 months in the 30 mg group, and 32.0 months in the 40 mg group. RESULTS: The overall one and two year recurrence rates were 19% and 24% in the 30 mg group, and 12% and 22% in the 40 mg group, which was not significantly different (p>0.05). Most of the side effects were mild and transient. Moreover, the rates of the individual side effects were not statistically different in the two groups. CONCLUSION: Our comparison of 30 mg and 40 mg intravesical MMC instillation showed no difference in either response or side effects. Thus, we tentatively conclude that we can use 30 mg instead of 40 mg as an intravesical MMC instillation dose.