The effect of preoperative bladder instillation (POI) and periodic prophylactic bladder instillation (PPI) of anticancerous drugs was evaluated in connection with the prevention of recurrent bladder tumors after surgery. A total of 191 patients with pTa or pT1 tumors including patients submitted to TURbt and partial cystectomy from January, 1967, to December, 1980, was chosen for the study. They were divided into the following 4 groups: Group A (49 cases) was treated with PPI and POI, Group B (11 cases) with PPI but not POI, Group C (46 cases) with POI but not PPI and Group D (85 cases) with neither PPI nor POI. POI was performed three times a week for a total of 20 applications of anticancerous drugs from two months before surgery. PPI was performed twice a month from one month after TURbt or parital cystectomy with a combination of 20mg of Mitomycin C and 1,000mg of 5-FU. The non-recurrence rate in these 4 groups was estimated by the actuarial method. The 3 year non-recurrence rates in Groups A,B,C and D were 95.4%, 90.9%, 44.0%, and 45.6%, respectively. The 5 year non-recurrence rates in Groups A,B,C and D were 82.4%, 81.0%, 32.0% and 35.1%, respectively. It is presumed from our study that PPI was effective in preventing the recurrence of bladder tumors. In comparison, POI showed a very limited effect and only in the first two years after surgery. No carcinogenic action on the bladder epithelium was observed from the topical use of Mitomycin C.