A 39-year-old woman with Sipple's syndrome and a malignant pheochromocytoma (PHE) is presented. She is a member of a big family with this syndrome and had undergone bilateral, subtotal adrenalectomy because of bilateral PHE six years prior to the present admission. In 1989, a small nodule was identified in her right thyroid lobe by ultrasonography and was suspected to be a medullary thyroid carcinoma (MTC). Further examinations revealed the coexistence of multiple lung and liver masses. These tumors were considered to be metastases of PHE because of the increased urinary excretion of catecholamines as well as extremely high catecholamines both in the hepatic venous blood and in the cystic fluid of the liver tumor even though there was no apparent recurrences of PHE in the adrenalregion. After surgical removal of the MTC in August, 1989, she was given 3.7 GBq of ¹³¹I-metaiodobenzyl guanidine (¹³¹I-MIBG) infusions twice-in October, 1989 and in August, 1990. ¹³¹I-MIBG showed a strong accumulation in the lung and liver masses on both occasions. Periodic increases in blood pressure and tachycardia with an excessive catecholamine secretion were observed over two weeks after the first treatment. However, this treatment was not effective in reducing urinary catecholamines nor the size of the metastatic tumors. She, therefore, underwent chemotherapy with cyclophosphamide, vincristine and dacarbazine in 1991, which slightly but significantly reduced urinary excretion of catecholamines and the size of the lung tumors. She had clinically stable period for one year after the treatment but rapid growth of these metastatic tumors occurred afterwards and she died in August, 1992. Unlike previous reports, ¹³¹I-MIBG therapy for malignant PHE was not as effective as expected but the patient showed a partial response to the combined chemotherapy.