A prospective randomized study was conducted to evaluate the efficacy of prophylactic intravesical instillation of tetrahydropyranyladriamycin (THP) following complete transurethral resection (TUR) of superficial bladder cancer. A total of 80 patients were randomized into "THP" or "control" group. In the THP group, 20 mg of THP dissolved in 40 ml saline (or 5% dextrose) was administered intravesically once a week for 10 weeks, starting from about 7 days after TUR. In the control group, 40 ml saline was given with the same schedule. The patients were followed up by cystoscopy and urinary cytology every 3 months. The number of evaluable patients was 36 for the THP group and 37 for the control group. The non-recurrence rates in the THP group and control group were 79.4% versus 63.2% at 1 year and 69.8% versus 47.4% at 3 to 5 years, respectively. These figures were not statistically significant. However, THP instillation significantly reduced tumor recurrence rates for multiple tumors, and also tended to decrease recurrence rates for primary and pT1 tumors. Adverse effects were observed in 53.6% of the patients in the THP group, but they were tolerable. Our results suggest that intravesical THP instillation would not be effective for all patients with superficial bladder cancer. Further study is warranted in a selected group of patients.