SINCE the recognition of medullary carcinoma of the thyroid as a histologic entity, its distinctive biological and clinical features have also become apparent.1,2 Adequate surgical treatment requires an awareness of its tendency to simulate certain behavioral characteristics of both papillary and poorly differentiated carcinoma, to involve both lobes of the thyroid, to metastasize to cervical nodes, to metastasize to distant sites, to resist therapeutic measures other than surgery, to be associated with other endocrine lesions, and to occur as an autosomal dominant characteristic in some families. For these reasons, an early, aggressive surgical approach, including total thyroidectomy and neck dissection, modified in accordance with operative findings, is indicated. Clinical Material Our experience with medullary carcinoma of the thyroid includes 18 patients seen during the past 12 years. It is biased to some extent in that ten of the 18 patients are members of two families with an inherited trait