Iodine [(131)I] tositumomab, administered in combination with unlabelled tositumomab, is a novel radioimmunotherapeutic regimen that targets the CD20 antigen present on normal and malignant B-cells. The efficacy and safety of the non-myeloablative regimen has been demonstrated in follicular and transformed follicular lymphoma over the last decade in a series of clinical studies, culminating in FDA approval in June 2003. In patients with relapsed or refractory disease some remissions have proven to be durable, and frequently longer in duration than previously administered chemotherapeutic agents. As initial therapy for advanced stage follicular lymphoma, response rates are particularly impressive. Toxicity has been principally haematological, with a single nadir at 4-6 weeks post-therapy. Administration is free of many of the side effects of conventional chemotherapy, although concerns about the long term risk of therapy related myelodysplasia persist. The challenge now comes from deciding the correct place of iodine [(131)I] tositumomab in treatment algorithms for follicular and other ''indolent'' lymphomas. Sequential administration after chemotherapy is being actively investigated, as is its role in myeloablative therapy. Issues of cost-benefit aside, it is a significant development in the therapy of these ''chronic'' malignancies.