Radioimmunotherapy (RIT) is a promising emerging therapy for non-Hodgkin's lymphoma and may ultimately prove useful in the treatment of other tumors. The most extensively investigated RIT agent is tositumomab and iodine I 131 tositumomab (Bexxar; Corixa Corp, South San Francisco, CA, and GlaxoSmithKline, Philadelphia, PA) which has been administered to over 1,000 patients during the past 9 years. As with most drugs, there is considerable interpatient variability in the clearance rate (or total body residence time) of radioimmunoconjugates. The clearance rate of iodine I 131 tositumomab in clinical trials has varied by as much as five-fold. The advantage of RIT with iodine-131, which emits both gamma photons and beta particles, is that by scanning it allows for the determination of the patient-specific total body residence time by the administration of a trace-labeled dose of the radionuclide (ie, dosimetric dose). By administration of the dosimetric (trace-labeled) dose, and determination of the patient's residence time (a measure of how long the radionuclide is retained in the body), the therapeutic dose can be precisely adjusted to maximize the therapeutic effect and minimize toxicity. Tositumomab and iodine I 131 tositumomab is a specific therapeutic at two levels: first, it specifically targets the tumor, delivering a log or more radiation to tumor compared with the rest of the body; and second, the administered dose of radioactivity is patient-specific. The paradigm of a targeted drug with a patient-specific dose may become more routine as targeted therapies are further developed along with better assays to directly measure drug levels. For the present, whole-body dosimetry is routinely applied for RIT with tositumomab and iodine I 131 tositumomab and has proven to be a reliable method to determine the patient-specific maximally tolerated therapeutic radiation dose to maximize efficacy while minimizing organ and bone marrow toxicity.