The potential concentration of I 131-Meta-iodo-benzylguanidine (MIBG) in pheochromocytoma, and the successful application in diagnosis and therapy of pheochromocytoma, has led to its use in therapy in other tumours derived from the neural crest. In neuroblastoma, the concentration of MIBG is as reliable as it is in pheochromocytoma. 18 patients with a neuroblastoma were treated, leading to two complete remissions, seven partial remissions, two no change, and two progressive disease; one patient was lost for follow-up. Six adults were treated, three with a carcinoid, two malignant pheochromocytoma and one medullary thyroid carcinoma. Although follow-up is still short, preliminary results of therapeutic use of I 131-MIBG indicate that this treatment modality may be effective.