Previous reports show elevated carcinoembryonic antigen (CEA) levels in sera from patients with medullary carcinoma of the thyroid (MCT). It was studied whether tumors now recognized as apud(amine precursor uptake, decarboxylase)omas other than MCT can produce the antigen because of the common origin of the tumors in the neural crest. Sera from patients with sporadic MCT and Sipple's syndrome were collected along with sera from patients with other types of apudomas: pheochromocytoma (11), neuroblastoma (5), pancreatic islet-cell tumor (2) and parathyroid adenoma (2). CEA was measured by radioimmunoassay. Increased levels of CEA were found in patients with MCT, either sporadic or Sipple's syndrome. Other apudoma patients showed normal or only slightly increased CEA levels. CEA production apparently is not a feature common to all apudomas. The specificity of CEA is not affected and its assay is a valuable procedure for the diagnosis and follow-up of MCT.