The results obtained using 131I-MIBG to diagnose different neuroendocrine tumors in 17 patients are discussed. In 8 of the neuroblastomas studied the examination gave a wide spectrum of scintigraphic results according to the various stages of evolution of the tumors, thus supplying additional useful information for metabolic treatment. Contradictory data were obtained in 5 cases of medullary cancer of the thyroid. The overall results were unexpectedly negative for 2 carcinoid tumors and, as expected, negative for 2 ganglioneuromas. These data confirm that 131I-MIBG is a non invasive, reliable and easy-to-perform technique which is no longer limited to the diagnosis of pheochromocytomas, but can also be used for the diagnosis of other tumors derived from neural crest cells.