Between 7/3/80 and 5/7/86 we gave 32 of our neuroblastoma patients 62 diagnostic doses of metaiodobenzylguanidine (MIBG) and 12 patients 20 treatment doses. Our conclusion from our diagnostic dose studies is that MIBG should be used for staging the extent of neuroblastoma before therapy is started, because it may change the proposed staging and therapy. In MIBG therapy for neuroblastoma, our criteria for agreeing to treat a patient are based on calculations from a 4-day tracer dose study that assures that the patient will receive from his first therapy dose a tumor dose of at least 2,000 rads/100 mCi, with a total body dose of not greater than 200 rads. Under these circumstances in children, the blood dose has been about 50 rads. The platelet count falls routinely with a 150-rad whole-body dose but never to dangerous levels. We have delivered tumor doses of 7,000-34,600 rads on the first dose using 150-215 mCi. We have had objective regressions (as shown by before and after CAT scans) of 30-59% in volume of the principal tumor mass in 3 of the first 12 patients treated. All patients had Grade IV neuroblastoma with extensive previous surgery, radiation, and chemotherapy, with and without previous bone marrow transplants. MIBG therapy was most effective in patients with slower-growing tumors for whom initial treatment doses were 200 mCi or more.