Objective Iodine-123 metaiodobenzylguanidine (I-123 MIBG) scintigraphy is gradually replacing I-131 MIBG scans in the diagnostic workup of neuroblastoma. High-dose I-131 MIBG, however, is commonly used for subsequent therapy in patients with proven MIBG-avid lesions. The objective of this study was to compare the sensitivities of pretherapy I-123 MIBG and post-therapy I-131 MIBG scans for detecting metastatic lesions of neuroblastoma and determine the suitability of post-therapy scans for detecting new metastases. Materials and methods Pretherapy I-123 MIBG scans and post-therapy I-131 MIBG scans of 126 patients with neuroblastoma were analyzed retrospectively and the number of detected lesions was compared. Results In 70 patients (55.6% cases), the pretherapy and post-therapy scans were concordant, showing similar MIBG-avid foci. In the remaining 56 patients (44.4% cases), the post-therapy I-131 MIBG scans revealed additional lesions (i.e. a total of 716 lesions) compared with pretherapy I-123 MIBG scans (only 532 lesions). All lesions detected on pretherapy I-123 MIBG scans were revisualized on the post-therapy I-131 MIBG scans, the latter also revealing 184 new MIBG-avid lesions. Conclusion Post-therapy I-131 MIBG scans demonstrate new lesions in almost half of the patients when compared with pretherapy I-123 MIBG scans and therefore are essential to delineate the disease extent. Nucl Med Commun 33:1134-1137 (C) 2012 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.