Iodine-131 metaiodobenzylguanidine (MIBG) has shown effectiveness as a systemic radiotherapeutic agent in neuroblastoma. The authors postulated a likely dose-related relationship of MIBG sensitivity when it was administered for neuroblastoma detection. They studied this relationship in neuroblastoma patients who underwent scanning after receiving diagnostic and therapeutic doses of MIBG in temporal proximity. Seven patients with stage IV disease received a total of 14 therapeutic administrations of I-131 MIBG (150-350 mCi [5,550-12,950 MBq]/m2 per treatment). Posttherapy scans were obtained at 3 and at 5-7 days. Diagnostic MIBG scans had been obtained no more than 4 weeks before the start of therapy. Use of diagnostic MIBG scanning led to underestimation of the tumor burden by 50% compared with use of posttherapy scanning. This difference may be an important consideration in selecting therapeutic strategies for individual patients. It further suggests that use of much larger diagnostic doses of MIBG is a rational strategy in histologically confirmed cases of advanced disease.