High values of carcinoembryonic antigen were detected in the blood of patients with medullary carcinoma of the thyroid (MCT). Levels of the antigen and calcitonin, the hormone which is specifically secreted by these tumors, are positively correlated. Stimulation of calcitonin secretion had no effect on carcinoembryonic antigen levels. Though the antigen assay is nonspecific for MCT it may improve the diagnosis and follow-up of MCT patients.