The syndrome of familial medullary thyroid carcinoma (MTC), pheochromocytoma, and parathyroid hyperplasia is inherited as an autosomal dominant trait, and is characterized by development of bilateral and multicentric thyroidal and adrenal medullary tumors. One of the earliest manifestations of adrenal medullary hyperfunction in patients with this syndrome is an increased ratio of epinephrine to norepinephrine in urine. In order to define the morphologic correlates of these early catecholamine abnormalities in a large kindred with familial MTC, a morphometric analysis based on a point-counting system to asses adrenal medullary volume was undertaken. These studies clearly revealed adrenal medullary hyperplasia as reflected by a two- to three-fold increase in medullary volume and weight as compared to age- and sex-matched controls. The increase in total medullary mass resulted from diffuse and multifocal modular proliferations of adrenal medullary cells primarily within the head and body regions of the glands. These results support the hypothesis that the pheochromocytomas in patients with familial MTC may, in fact, represent extreme degrees of nodular hyperplasia of the medulla.