CEA is a molecule produced by a large number of malignant and benign tissues. Measuring levels of CEA circulating in the blood by radioimmunoassay can be used in the management of cancer patients. Because of high false positive and false negative percentages in normal populations, it has not been useful in screening for malignancy. However, in several types of cancer patients the test has been shown to be of considerable clinical value. Elevated CEA levels indicate a poor prognosis in patients with primary colorectal cancer, primary pancreatic cancer, primary breast cancer, and primary lung cancer. Serial CEA titers obtained following cancer treatments can be used to monitor the therapy. CEA can assess the adequacy of surgical removal of a primary colon or rectal cancer, monitor responses to chemotherapy, and assess response to radiation therapy. The greatest clinical impact of CEA has been in the detection of recurrent colon or rectal cancer following surgical resection of the primary malignancy. Early detection of recurrence, when combined with reoperative second-look surgery, may result in 30% long-term survivors.