Two hundred and twenty-eight patients who were treated for carcinoma of the lung were followed and their plasma CEA levels assessed at intervals during the course of the disease. In addition, plasma samples were taken from 487 healthy blood donors for comparison as a control. CEA assay is not selective or specific enough, at this time, to be used for screening purposes even though 68% of the patients who have lung cancer will have an elevated concentration of CEA regardless of the histological cell type. In patients with plasma levels of CEA above 15 ng/ml the prognosis is uniformly poor. CEA in the authors' view does have value as a prognostic marker capable of suggesting the successful resection of a tumor and to a lesser degree confirming the clinical objective response to the radiotherapy or chemotherapy. It was found that the presence of CEA was not necessarily related to the volume of the tumor or the site of organ metastasis, but reflects the metabolic properties and characteristics of the individual tumor as it occurs in the patient.