Seven patients with neuroblastoma (six children and one adult) were treated with therapeutic doses of high specific activity 131I-metaiodobenzylguanidine (131I-MIBG). Six patients were in stage IV and unresponsive to conventional treatment. One patient, in stage III, was treated at diagnosis, an approach never previously reported. Single doses of 131I-MIBG varying from 70 to 184 mCi split into two parts were administered by slow i.v. infusion (4 to 8 hours) at 2- to 4-day intervals. The following results were obtained in the six evaluable patients: two patients showed transient stabilization of the disease; three had an objective response, with shrinking of the primary tumor and/or regression of the metastatic lesions. Of these three patients, two suffered relapses at 2 and 7 months, respectively, from the first course of MIBG. The third patient, in whom the residual disease almost completely disappeared following MIBG therapy, is still alive in complete remission after autologous bone marrow transplantation with a follow-up of 14 months. The single patient treated at diagnosis showed a dramatic response to a relatively low dosage of MIBG, with histologically proved disappearance of the tumor mass. Our data indicate that MIBG may be useful in the treatment of neuroblastoma unresponsive to conventional chemotherapy. The complete response observed in the patient treated at diagnosis suggests that the full potentiality of MIBG therapy should be explored in untreated patients.