I 131-metaiodobenzylguanidine (MIBG) is an aralkylguanidine with certain structural similarities to norepinephrine (NE). It is concentrated, stored, and released from chromaffin granules in a manner almost identical with that of NE. It will image the enlarged adrenal medullae of adrenal medullary hyperplasia when the CAT and NMR scans are normal. It is more sensitive in detecting extra-adrenal pheochromocytomas than CAT and NMR imaging. Because 46% of our 176 patients with histopathologically proved "benign" pheochromocytomas (pheos) have developed demonstrable metastases, with or without elevated plasma and urinary catecholamines, we now image all patients with "benign" pheos yearly. As of January 22, 1986 we had treated 28 patients with malignant pheos 71 times with MIBG. As of July 24, 1986, we had given 34 neuroblastoma patients 55 tracer doses. In some cases MIBG demonstrates more neuroblastoma than all other imaging modalities and this is helpful in staging. We have had 30-50% objective regressions in neuroblastoma tumor mass in 3 out of the first 12 patients treated. These three patients had slower-growing tumors and a lower body burden than the nonresponders. We also record the sensitivity of MIBG imaging of neuroendocrine tumors other than pheos and neuroblastomas.