Serum levels of sialyl-Tn antigen (STN) were measured using a one-step radioimmunoassay kit in 257 patients with digestive cancers, 121 patients with benign digestive diseases, and in 64 healthy controls. With 45 U/ml regarded as the cutoff value, the positive rates of serum STN in digestive cancers were as follows: pancreas 40.0%, stomach 28.1, colon and rectum 27.8, biliary tract 25.0, liver 7.1, and esophagus 0%. In benign digestive diseases, the positive rate of STN was low (4.1%). In gastric and colorectal cancers, simultaneous measurements of STN and carcinoembryonic antigen (CEA) revealed that the positive rates of STN, CEA, and STN and/or CEA were 28, 42, and 55%, respectively. There was no significant correlation between STN and CEA in these cancers. STN may be a useful serum marker for digestive cancers, especially gastric and colorectal cancers.