Fourteen patients with chromaffin tumors were treated with [131]metaiodobenzylguanidine (131I-MIBG); 13 of them suffered from malignant and one from benign pheochromocytoma. In all patients clinical symptoms were improved. In some of these patients tumor shrinkage was observed. In one patient surgery of all tumor tissue was made possible by 131I-MIBG treatment. As shown in one patient, close follow-up is necessary in any case because remnant tumor tissue may start growing again after a long period of rest.