A patient with advanced neuroblastoma who had failed chemotherapy presented with a large abdominal mass and virtually total bone marrow replacement by tumor on repeated marrow biopsies. She was considered a candidate for a Phase I 131I-3F8 radioimmunotherapy trial, (MSKCC 89-141A). As a potential aid to treatment planning, a test dose of 124I-3F8 was injected and the patient was imaged over the 72 hr postinjection using two BGO based PET scanners of different designs. Time activity curves were obtained, and the cumulated activity concentration of radiolabeled 3F8 in tumor was determined. Based on MIRD, an estimated radiation absorbed dose for 131I-3F8 was 7.55 rad/mCi, in the most antibody avid lesions. Because of low uptake and unfavorable dosimetry in some bulky tumor sites, it was decided not to treat the patient with radiolabeled antibody. Positron emission tomography of 124I-labeled antibodies can be used to measure cumulated activity or residence time in tumor for more accurate estimates of radiation absorbed tumor dose from radioiodinated antibodies and can help guide management decisions in patients who are candidates for radioimmunotherapy.