Background. We examined the long-term effects of two chemotherapeutic agents, thiotepa and UFT, a 4 : 1 mixture of uracil and tegafur (a 5-fluorouracil pro-drug), in preventing postoperative recurrence of superficial bladder cancer. Methods. The subjects of the study were 153 patients with newly diagnosed superficial bladder cancer (Ϲ grade 2, Ϲ pT1b) who underwent transurethral resection of bladder tumors (TURbt) between November 1986 and March 1990. The patients were randomized into three groups, group 1 (intravesical instillation of thiotepa), group 2 (oral administration of UFT), and group 3 (no treatment). The initial treatment was maintained until the third recurrence or disease progression. The patients were observed until February 1997 and the findings analyzed statistically. Results. The non-recurrence rate during the 5-year period after TURbt was 85.2% in group 1, 40.3% in group 2, and 53.2% in group 3, with the rate in group 1 significantly higher than that in groups 2 and 3. Analysis of the nonrecurrence rate in relation to cancer grade or stage showed that patients with grade 2, stage pT1 tumors in group 1 had a significantly higher non-recurrence rate than patients with grade 2, pT1 in tumors groups 2 and 3. Multivariate analysis by Cox's proportional hazards regression model confirmed that prophylactic treatment was the most important factor affecting the non-recurrence rate. Conclusion. Intravesical thiotepa instillation after TURbt was useful to prevent the recurrence of bladder cancer in patients with grade 2 or pT1 tumor in a 5-year period.