Malignant pheochromocytomas, a group of tumors that include metastatic paragangliomas, often produce hypertension and episodic symptoms from secretion of norepinephrine and sometimes epinephrine. In addition, the tumors usually manifest progressive metastases. Blockade of alpha and beta adrenergic receptors will control blood pressure and symptoms, but reduction of the malignancy has been difficult to achieve. Meta-iodobenzylguanidine (MIBG) follows the pathways of norepinephrine and, when labeled with 131-I, will concentrate sufficiently in the pheochromocytoma to impart therapeutic radiation. More than 100 patients have received treatment with 131-I-labeled MIBG at multiple medical centers. Individual doses were 3.7 to 18.5 GBq (100 to >500 mCi), and many patients received several doses separated by a few months. Partial remissions, recorded as decreased tumor presence and tumor function, have been observed in one-third or more of the treated patients. However, complete remissions are rare, and recurrence/progression within two years is the rule. Toxicity was generally modest and temporary. Subsequent chemotherapy increased the benefits attained by 131-I MIBG, but, in a small series of patients, this combination did not further change the outcome. Nevertheless, selective radiation from 131-I MIBG or a similar radiopharmaceutical could play a valuable role in treatments that combine several types of attacks on this recalcitrant malignancy.