Thirteen surgical patients affected by colorectal cancer have been evaluated to study the effectiveness of a short-term preoperative therapy with interleukin-2 (IL-2). Seven patients have been treated for three days before surgery with subcutaneous administration of IL-2 (9.000.000 U.I. b.i.d.). Six patients have been treated with placebo and have been considered as control group. All the patients have been operated upon 36-48 hours after the last administration of IL-2 or placebo. The assays of CD-3+, CD-4+, CD-8+ e CD-56+ lymphocytes, evaluated preoperatively and 7 days after the operation, have showed a postoperative increase of CD8+ and CD56+ and a decreased ratio of CD4+/CD8+. The results of this study, although not conclusive, suggest a positive antitumoral response in patients treated preoperatively with IL-2. Further studies could be performed to evaluate the survival after such a treatment.