Medullary thyroid carcinoma (MTC) was first described as a distinct clinical entity by Hazard et al1 in 1959. Although the lesion was previously thought to be a variant of anaplastic thyroid carcinoma, these investigators appreciated that certain histologic characteristics distinguished it from other thyroid neoplasms. Actually, MTC is derived not from follicular or stromal elements but from neural crest cells, which in mammals are incorporated within the thyroid gland. In birds and fish these cells exist as a separate structure called the ultimobranchial body. In retrospect, it is surprising that MTC was not recognized sooner as being different from other thyroid malignant neoplasms because it has certain unique characteristics, including frequent involvement of both thyroid lobes, occurrence in a familial pattern with assocated endocrinopathies (multiple endocrine neoplasia [MEN] types IIa and IIb), the presence in the tumor of a substance with the histochemical staining properties of amyloid, and the