Estimation of CEA levels by the Z-gel method indicates that smokers, patients with limited lung cancer and patients with extensive lung cancer have higher values than nonsmoking controls. The CEA levels within each group are significantly different from one another. Use of CEA estimation for diagnostic purposes is limited because of the considerable overlap between normal controls and patients with cancer, the relatively low incidence of elevated values in patients with limited disease and the high incidence of false negatives (20%) even in patients with extensive disease. Elevated CEA values are associated with a poor prognosis and could be of clinical value as an addition to clinical staging to determine survival particularly for patients with extra-thoracic disease. Persistently high values in patients deemed clinically disease-free postoperatively are indicative of residual disease and a poor prognosis. If and when effective therapy for bronchogenic carcinoma becomes available, monitoring of CEA values may be useful in some patients as an early indication of release. Further studies are required to determine if the extraordinarily poor prognosis associated with marked elevations of CEA may be used as an additional criterion of inoperability in such patients.