The authors have studied 43 patients operated on for medullary thyroïd carcinoma. Plasma calcitonin was measured regularly in all patients, while carcinoembryonic antigen assay was performed in only 30 patients. Calcitonin assay was found to be useful for preoperative diagnosis of medullary carcinoma, and the level of plasma calcitonin appeared to be roughly correlated with tumor extension. After surgery, simultaneous assay of calcitonin and carcinoembryonic antigen was performed, in order to obtain more accurate information concerning the evolution and prognosis of the disease. In most cases in which no metastatic lymph nodes had been discovered at operation, the level of the two markers rapidly fell to undetectable values. It was observed that in patients with lymph node involvement, cervico-mediastinal radiation treatment did not change the slow and progressive evolution of the disease. However, a rapid increase in titre of carcinoembryonic antigen occurred simultaneously with the discovery of metastases, even when calcitonin levels did not dramatically change.