Metaiodobenzylguanidine (MIBG) has been used for the detection and treatment of neuroectodermal tumors, including neuroblastoma. We report our experience with 131I-MIBG used diagnostically in neuroblastoma patients with relapse. Thirty-eight studies were performed in 26 patients. There were 24 children (range 3 months-14 years) and two adults. While the study was found to be both sensitive and specific for the presence of disease, there are instances of discordance. False-negative studies were found with a markedly anaplastic tumor and with two mature ganglioneuromas. A bone lesion was negative with 131I-MIBG, but positive on bone scan. A biopsy confirmed the presence of neuroblastoma. Caution should be exercised when scanning pretreated patients, and perhaps with newly diagnosed patients as well.