131I-metaiodobenzylguanidine(MIBG) has been used for the diagnosis and treatment of neural crest tumors. We report our experience with this agent in 8 patients[l multiple endocrine neoplasia(MEN) type IIb; 2 malignant pheochromocytoma; 5 medullary thyroid carcinoma(MTC)]. The therapeutic procedure consisted of 30-200 mCi of 131I-MIBG administered by slow I.V. infusion, given at 3-6 months intervals. Cummulative activity ranged from 150 mCi to 410 mCi, in 1 to 4 courses. 131I-MIBG therapy resulted in significant disease free interval in 1 malignant pheochromocytoma(no measurable lesion)after sugery; complete hormonal and tumoral response in 2 MTC(1 MEN IIb); stable disease in 1 recurred pheochromocytoma(MEN IIb); stable disease but symptomatic improvement in 1 MTC; progressive disease in 1 malignant pheochromocytoma and 2 MTC. The patients who showed progression appeared to have large inoperable tumors or postoperative remnant tumors.