Effects of high activities of I 131 meta-iodobenzylguanidine (mIBG) were evaluated in nine children with advanced neuroblastoma. All patients had been previously heavily treated and had either primarily refractory disease or resistant relapse. Twenty-two doses of mIBG labeled with 1.3 to 4 GBq (35-108 mCi) of iodine 131 were administered. Three subjective effects, especially relief of pain, and two objective effects were observed. Transient blood pressure increase was observed once and did not recur after prolongation of the infusion time to 6 hours. A major side effect was bone marrow toxicity, essentially marked by thrombopenia, particularly severe in previously bone-marrow-transplanted patients.