Carcinoembryonic antigen (CEA) levels in the serum and colon tissue were evaluated in 83 patients with colon cancer. The mean concentration of serum CEA increased in parallel with Dukes' stages. CEA concentrations in the cancerous and normal colon overlapped. In the same patient, however, CEA of cancerous colon tissue was always higher than that of normal colon tissue. There was no difference in CEA concentrations in colon cancer between Dukes' A and B. However, in patients with Dukes' C stage, the concentration was higher than in those with Dukes' A and B. The detection of tissue CEA concentration as well as serum CEA is a useful marker for monitoring patients with colon cancer.