Carcinoembryonic antigen (CEA) is still the best marker both for primary diagnosis and post-treatment monitoring of patients with colorectal cancer. Monoclonal antibodies, especially CA 19-9 and CA 50 may give additional information whereas CA 125 seems to be of no value in patients with colorectal cancer. The sensitivity of CEA determination for Dukes' A carcinomas is as low as 30%, but increases to 85% for Dukes' D carcinomas. The best clinical benefit of CEA is in postoperative monitoring of surgically treated patients with colorectal cancer. The sensitivity and specificity for distant metastases are 85%. The sensitivity in the detection of local recurrence is low (40%) but the specificity is still high (80%). A high CEA level postoperatively strongly suggests either local recurrence or disseminated disease, but a negative value does not exclude their presence. If CEA is negative both preoperatively and one month postoperatively, CA 19-9 or CA 50 may be used in the monitoring of these patients.