Non-Hodgkin's lymphoma I-131 radioimmunotherapy data were analyzed to determine whether a predictive relationship exists between radiation absorbed doses calculated from biodistribution studies and doses derived from patient size. Radioactivity treatment administrations scaled to patient size (MBq/kg or MBq/m2) or fixed MBq doses do not produce consistent radiation absorbed dose to critical organs. Treatment trials that do not provide dose estimates for critical normal organs are less likely to succeed in identifying a clinical role for radioimmunotherapy.