It has been demonstrated that surgery may induce immunosuppression. This finding could influence the clinical course of surgically treated cancer patients. Moreover, preliminary experimental studies have shown that a preoperative injection of IL-2, whose importance in generating the antitumor immune response is well known, may neutralize surgery-induced immunosuppression. At present, however, it is still unknown whether preoperative IL-2-induced immune improvement in the postoperative period may influence the prognosis of surgically treated cancer patients. The present study was performed to evaluate the prognostic impact of IL-2 presurgical therapy in advanced colorectal cancer patients. The study included 50 colorectal cancer patients, Dukes’ stage D, who were randomized to be treated with or without IL-2 pre-operatively (18 • 106 IU/day subcutaneously for 3 consecutive days). After surgery, all patients underwent chemotherapy with 5-FU and folates until disease progression. Postoperative mean numbers of lymphocytes, T lymphocytes, natural killer cells and activated lymphocytes were significantly higher in IL-2-treated patients than in controls. Moreover, the percent of lymphocytic and/ or eosinophilic tumor infiltration was significantly higher in IL-2 group than in controls. Finally, both survival curve and the percent of survival at 1 year were significantly greater in patients pretreated with IL-2 than in controls. This clinical trial demonstrates that preoperative IL-2-induced neutralization of postoperative lymphocytopenia is associated with a prolonged survival time in advanced colorectal cancer patients.