Elevated interleukin-10 (IL-10) and IL-6 serum levels in advanced gastrointestinal cancer patients have been shown previously. To investigate the behavior and the prognostic role of IL-10 and IL-6 serum levels in gastric and colon cancer patients undergoing surgery, we studied the relationship between these cytokine levels and surgical radicality and outcome. Seventy-eight patients with gastric or colon cancer were admitted to the study, and 50 underwent radical surgery. Cytokine serum levels were measured by ELISA the day before surgery and 16 days after surgery. Circulating levels of IL-10 and IL-6 were found to be higher in cancer patients than in controls. Both IL-10 and IL-6 serum levels were demonstrated to be able to predict likelihood to perform radical surgery. IL-10 serum levels returned to normal in all but 8 radically resected patients. These 8 patients had tumor recurrence. In contrast, IL-6 serum levels were shown to significantly decrease in all patients but not to normalize regardless of the radicality of the operation. On multivariate analysis, basal IL-10 serum levels were found to be among the variables significantly affecting the disease-free survival rate. Stepwise regression selected tumor stage, number of metastatic resected nodes, and basal IL-10 serum level as the best combination of variables for prediction of likelihood of tumor recurrence. Preoperative IL-10 serum levels may be a useful marker to predict likelihood of performing radical surgery. Abnormally high postoperative IL-10 values negatively affected disease-free survival and tumor recurrence. IL-6 serum levels were found to have a more limited prognostic role.