It has taken approximately 18 years to define the clinical utility of carcinoembryonic antigen (CEA) as a marker in patients with colorectal cancer. Hopefully, the use of CEA as a prototype and the knowledge of the many clinical studies designed to define its use in patients with colorectal cancer will help avoid the need for 18 years of studies in determining the biologic and clinical applicability of the many promising new candidate tumor markers now being developed. In this paper we will review our own studies which have helped to define the clinical utility of CEA in the care of patients with colorectal cancer.