The medullary thyroid carcinoma (MTC) is a rare neoplasia occurring during childhood. At present time the molecular examination of the proto-oncogen RET, related to syndromes of multiple endocrine neoplasia (MEN II) and familial medullary thyroid carcinoma (FMTC) to allows identify patients with risk of suffering of medullary thyroid carcinoma in early ages, before the disease becomes clinically pronunced. Children with familial antecedents of MEN II or FMTC were biochemically (pentagastrin-stimulated) and genetically studied with the purpose of determining the risk of developing a MTC and in order to assess the possibilities of making a prophylactic thyroidectomy.