Medullary carcinoma of the thyroid (MCT) is characterized by high circulating levels of calcitonin (CT) and of carcinoembryonic antigen (CEA), these markers enable the diagnosis of the tumour, the assessment of the efficacy of treatment and the detection of metastases at a subclinical stage. In 130 patients with a primary tumour and/or metastases, CT level was high. In 120 of them (92%), CEA level was also high. There was a positive correlation between levels of CT and of CEA, and both assays should be included in the preoperative examinations. Following removal of the tumour, normalisation of CEA levels takes several weeks, in contrast to CT where normalisation is rapid. Finally, these estimations may be used to detect familial forms at a subclinical stage. This enabled us to detect 16 new cases of MCT amongst 77 subjects studied.