Five cases of malignant pheochromocytoma (3 men and 2 women, aged 26-43 years) were treated with [131I]metaiodobenzylguanidine (131I-MIBG). One patient had a voluminous adrenal tumor and multiple distant metastases; two patients had a recurrent tumor; two others a post-surgical residual tumor. The therapeutic procedure essentially consisted of single doses (2.6-7.4 GBq) of 131I-MIBG administered by slow i.v. infusion, given in several therapeutic courses at 1-5 month intervals. The treatment resulted in a complete response in one case with residual tumor and in a partial response in the case with disseminated disease. Two cases showed stabilization of the disease, whereas therapy was ineffective in the fifth case. Nevertheless, pain relief was observed in this patient. The treatment had a very low toxicity and was well tolerated by all patients.