The newly developed radiopharmaceutical, 131I-metaiodobenzylguanidine (131I-MIBG), has been shown to be efficacious for the location of intra- and extra-adrenal, primary pheochromocytomas and metastatic, malignant pheochromocytomas (11.4% false-negative and 1.8% false-positive in patients with proven pheochromocytomas). Preliminary experience in selected patients with malignant pheochromocytoma suggest that therapy using large doses of I31I-MIBG results in partial tumor regression and improvement in catecholamine hypersecretion in some cases.