Thirty-one patients were studied to evaluate the prognostic value of both calcitonin (CT) and CEA levels determined after the initial treatment in medullary thyroid carcinoma (MTC). Twenty-seven patients were evaluated three to nine months after initial treatment and four others afterwards. The CT and CEA levels were significantly higher and the survival rate lower in the eight patients with residual clinical disease as compared to the 19 patients in complete clinical remission. In patients in complete clinical remission, 11 had elevated CT level after treatment, and all had initial lymph node involvement. Five of these 11 relapsed and one died. None of the eight patients with normal CT levels after treatment relapsed. CEA levels were always abnormally high when patients relapsed. Fourteen patients in complete remission with high CT levels were followed for more than four years. Six had normal CEA levels and no relapse was observed. Eight of the 14 had pathological CEA levels and six of eight relapsed: five of these six patients presented CEA elevation from six to 36 months before the clinical relapse. In two of these six patients, a venous catheterization sampling method demonstrated infra clinical local recurrence. In two patients with liver metastases, the time course changes of CT and CEA levels were different and CEA appeared to be a more sensitive tumor marker than CT. These data are consistent with previous data concerning the values and limits of CT level for the management of MTC. Furthermore, this study demonstrates the prognostic significance of CEA determination in MTC. CEA appears to be a sensitive selective tumor marker capable of defining a high-risk subgroup.