The level of carcinoembryonic antigen (CEA) is often elevated in the serum of patients with cancer. This article reviews the clinical usefulness of this observation. Carcinoembryonic antigen is not useful for detecting asymptomatic cancer; its sensitivity and specificity are not high, particularly for early stages of disease, so in populations with low prevalence of disease there are many false-positive and false-negative results. Similarly, the antigen level cannot, by itself, provide enough diagnostic certainty to confirm or rule out suspected cancer. For some cancers, antigen levels at the time of diagnosis provide more precise prognosis than staging alone, but this information does not lead to more effective treatment. Serial measurement of CEA levels after surgery in patients with colorectal cancer can detect recurrences early, but few lives can be saved by this approach. Thus, CEA assays provide accurate information about some aspects of cancer but rarely lead to better outcomes for patients.