Medullary carcinoma of the thyroid gland is a feature of multiple endocrine neoplasia, type IIb (MEN IIb). The cancer frequently gives rise to metastases in early life. Marfanoid habitus and virtually pathognomic mucosal ganglioneuromas, often situated on the tongue, enable early diagnosis. These stigmata should alert the clinician to the possibility of MEN IIb before medullary carcinoma is clinically manifest. We now believe that it is reasonable to perform a total thyroidectomy in children with the typical physical appearance of this syndrome regardless of age since medullary carcinoma of the thyroid gland appears in almost every case. Calcitonin, a hormone secreted by the C-cells, serves as a plasma tumor marker. Intravenously administered, pentagastrin is a potent secretagogue which is very useful in the early diagnosis of either primary or recurrent medullary carcinoma. With this pentagastrin test, a laboratory screening program is possible allowing the clinician, specialist, to recognize the syndrome.