The result of intravesical instillation with interleukin-2 (IL-2) for preventing the recurrence of bladder cancer and discussion of its mechanism was reported. 20 patients with histologically confirmed bladder transitional cell carcinoma were investigated. 2 000 u IL 2 was intravesically instillated every day for six days after resection of the neoplasm. The serum level of tumor necrosis factor (TNF) was significantly increased in 17 patients after IL-2 therapy. Recurrence occurred in 4 patients during the follow-up period of 10-18 months. The basic level of serum TNF was low and was not increased or even decreased after IL-2 in these patients. On the contrary, recurrence did not occur in another 5 patients with low basic level of TNF, but it was significantly increased after treatment of IL-2. TNF played a key role in preventing the recurrence of bladder cancer. The patients who had low basic level of TNF, which did not markedly increase or even decrease after IL-2 therapy were at high risk of recurrenc The result of intravesical instillation with interleukin-2 (IL-2) for preventing the recurrence of bladder cancer and discussion of its mechanism was reported. 20 patients with histologically confirmed bladder transitional cell carcinoma were investigated. 2 000 u IL 2 was intravesically instillated every The serum level of tumor necrosis factor (TNF) was significantly increased in 17 patients after IL-2 therapy. Recurrence occurred in 4 patients during the follow-up period of 10-18 months. The basic level of serum TNF was low and was not increased or even decreased after IL-2 in these patients. On the contrary, recurrence did not occur in another 5 patients with low basic level of TNF, but it was significantly increased after treatment of IL -2 TNF played a key role in preventing the recurrence of bladder cancer. The patients who had low basic level of TNF, which did not markedly increase or even decrease after IL-2 therapy were at hig h risk of recurrenc