The value of carcinoembryonic antigen (CEA) assay for diagnosis and follow-up in patients suffering from medullary carcinoma of the thyroid (MCT) was investigated in a large sample (106 cases). High levels of CEA were found in 84% of patients suffering from either the sporadic or the familial form of the disease. Levels of CEA and calcitonin (CT) are significantly and positively correlated. Removal of tumoral tissue is followed by a decrease in both CEA and CT levels. High levels of CEA were also observed in the parents of patients suffering from the familial form of MCT. These patients were operated on and MCT was confirmed histologically. The limitations of the use of CEA assay in the diagnosis of MCT are discussed.