PURPOSE: We evaluated the efficacy of perioperative mitomycin C (MMC) instillation to improve subsequent bacillus Calmette-Guérin (BCG) instillation efficacy in intermediate and high risk patients with non-muscle invasive bladder cancer (NMIBC). MATERIALS AND METHODS: From November 2004 to May 2006, 51 patients with intermediate or high risk NMIBC were enrolled in this prospective randomized trial. In group A, patients were treated with perioperative MMC (40 mg MMC in 40 mL saline was administered within 6 hours of surgery) followed by delayed (at least 15 days from surgery) BCG instillations (once a week for 6 weeks, 5 x 108 colony-forming units in 50 mL saline). Patients in group B were treated with delayed BCG instillations alone. The primary end points were recurrence-free interval and recurrence rate. RESULTS: There were 25 and 26 patients in groups A and B, respectively. Median follow-up was 41.3 months (range 8 to 64) in group A and 40.9 months (range 6 to 68) in group B. Recurrence rate was 36% (9 of 25) and 19.3% (5 of 26) in group A and B, respectively (p = 0.052). Median time to the first recurrence was 8 months in group A and 7 months in group B (p = 0.12). CONCLUSIONS: The present study showed no statistically significant difference in terms of recurrence rate and median time to first recurrence between intermediate or high-risk patients with NMIBC who were treated with early single dose instillation of MMC plus delayed BCG and those who were treated with only BCG.