To examine the prognostic value of assessing the concentration of circulating sialyl Tn antigen (STN) after surgery, we determined serum STN levels in peripheral venous blood (designated "p-") in 308 colorectal cancer patients and what we have termed the "d-p gradient" (obtained by subtracting the serum concentration in peripheral venous blood from that in the tumor's drainage venous blood) in 144 patients. The prognostic value of STN and carcinoembryonic antigen (CEA) was compared. Patients were divided into low or high p-antigen groups and low, intermediate, or high d-p gradient groups. Univariate and multivariate analyses revealed that high STN d-p gradient, high p-CEA, or high CEA d-p gradient were each independent variables for poor patient outcome after surgery, separate from stage. In conclusion, a high STN d-p gradient was a predictor of poor outcome after resection for colorectal cancer, while p-STN was not independent of stage.