Metaiodobenzylguanidine (MIBG) is actively concentrated in adrenergic neuroendocrine tissues and tumors by an active, energy- and sodium-dependent, high-affinity, saturable mechanism. This has proved successful, when labeled with 131-I or 123-I, in scintigraphically depicting pheochromocytomas and neuroblastomas. For imaging purposes 123-I has multiple advantages over 131-I; the gamma ray energy is ideal for modern instruments, the decay by electron capture limits the particulate emissions, and the short half-life reduces the radiation burden. It is thus possible to use doses of 123-I-MIBG 20 times as large as doses of 131-I-MIBG with equivalent absorbed radiation doses. Disadvantages of 123-I include the cost and difficulties in the regular delivery of this short-lived radionuclide. For most imaging purposes 123-I-MIBG is the optimal agent if logistical problems in its supply can be overcome.