Toxic effects from 131I-meta-iodobenzylguanidine (131I-MIBG) treatments of neuroblastoma in six patients were recorded. The toxicity was largely confined to the hematologic system where circulating leukocytes and platelets regularly declined after each dose of 131I-MIBG; the values reached nadirs between three and seven weeks and recovered slowly over subsequent weeks. Prior bone marrow transplantation and infiltration of bone marrow by neuroblastoma appeared to make the hematologic system more vulnerable to the radiation. Dosimetry revealed greater absorbed radiation by the whole body than by the blood and bone marrow. These observations are explained by a relatively rapid exit of 131I-MIBG from the blood to other tissues (but not to the bone marrow). Since treatment of an aggressive and lethal tumor such as neuroblastoma should be pushed to a degree of toxicity, careful dosimetry in each case will be necessary as a guide to reach the point of maximally tolerable toxicity.