131I-metaiodobenzylguanidine (131I-MIBG) is used for localization procedures in patients with phaeochromocytoma. In high doses and concentrations the same radiopharmaceutical agent can be used for treatment of adrenomedullary tumors. In 4 of our patients, treatment with 131I-MIBG improved the clinical symptoms. In 2 patients with a multilocular phaeochromocytoma, a tumor reduction of some 61% after a total dose of 415 mCi, and of about 10% in the second patient after 365 mCi 131I-MIBG, was measured by computed tomography. In a unilocular intra-adrenal phaeochromocytoma treated with 131I-MIBG, initial clinical improvement was followed by an obstruction of the large abdominal vessels by the growing tumor. There was no close correlation between therapeutic effect and catecholamine secretion. No severe side effects were observed. This new nuclear medical treatment appears to be a reliable therapeutic method for malignant phaeochromocytoma.