A retrospective study was performed on 101 consecutive medullary thyroid cancer (MTC) patients who underwent at Institut Gustave-Roussy (IGR) total thyroidectomy with central and bilateral lymph node dissection. At histology, lymph node metastases were found in 55% of patients. In sporadic MTC, lymph node metastases were observed in the central compartment in 50% of patients, in the ipsilateral jugulocarotid chain in 57%, and in the contralateral jugulocarotid chain in 28%. In hereditary MTC, lymph node metastases were identified in the central compartment in 45% of patients, in the ipsilateral jugulocarotid chain in 36%, and in the contralateral jugulocarotid chain in 19%. Contralateral lymph nodes were found in 37% of metastatic patients with an unilateral tumoral involvement of the thyroid gland. A strong association was observed between tumor size and lymph node involvement for both hereditary and sporadic MTC (P < 0.02). Permanent hypoparathyroidism occurred in 4% of patients and laryngeal nerve palsy in 5%. An undetectable calcitonin level was obtained after surgery in 61% of patients, in 95% of patients without lymph node metastases, and in 32% of patients with lymph node metastases. Among patients with lymph node involvement, undetectable calcitonin level was obtained in 57% of patients with less than or with 10 lymph node metastases and in 4% of patients with more than 10 (P < 0.01). 1) lymph node metastases occur early in the course of MTC; 2) the pattern of lymph node metastatic distribution in neck areas varied between patients and was not related to the thyroid tumor size; 3) contralateral lymph node metastases were observed even in patients with small thyroid tumor; 4) total thyroidectomy with central and complete bilateral neck dissection should be performed routinely in all patients with sporadic and hereditary MTC, even in those with small thyroid tumors-a contralateral neck dissection may be avoided only in sporadic MTC patients with unilateral involvement of the thyroid gland in the absence of central and ipsilateral neck involvement; and 5) the number of lymph node metastases was predictive of biological cure after surgery.