Carcinoembryonic antigen (CEA) is a 180 KDa glycosylphosphatidylinositol-anchored cell surface glycoprotein that belongs to the immunoglobulin superfamily. CEA is found in many types of cells but is associated with tumors and the developing fetus. Benign conditions that can increase CEA include smoking, infection, inflammatory bowel disease, pancreatitis, cirrhosis of the liver, and some benign tumors. Benign disease does not usually cause a CEA increase over 10 ng/mL whereas levels over 20 ng/mL before therapy are associated with cancer that has already metastasized. CEA is produced by 90% of colorectal cancers and contributes to the malignant characteristics of the tumor. Serum CEA level is the most widely used tumor marker for colorectal cancer as rising concentrations of CEA precede other clinical indicators by several months and are associated with an increased risk of relapse and poor patient outcome. If CEA is high before treatment, it should fall to normal after successful therapy. CEA levels are also elevated in many other cancers such as thyroid, pancreas, liver, cervix, and bladder.