Six patients with multiple endocrine neoplasia (MEN) types 2a and 2b were investigated to determine the spectrum of pheochromocytoma by scintigraphy. Iodine-131-metaiodobenzylguanidine (131I-MIBG), a new imaging agent which concentrates in adrenergic neurotransmitter vesicles, was administered at 0.5 mCi/1.7m2 and scintiscans were taken at 24 and 48 hours. Two normotensive patients with normal plasma and urinary catecholamines had no adrenal tracer uptake. One patient with a modest and intermittent increase only in urinary catecholamine metabolites showed faint adrenal images. Two other patients with increased plasma and urinary catecholamines showed bilateral adrenal imaging patterns. The sixth patient who had increased norepinephrine and epinephrine secretion showed bilateral asymmetrical adrenal images, findings that were corroborated at operation. Functional as well as anatomic evidence of adrenal medullary abnormalities in patients with MEN-2 syndromes are demonstrated by 131I-MIBG scintigraphy. Therefore, the procedure can be used to define the extent of abnormalities of the adrenal medulla in these patients.