Radioimmunotherapy consists of radiolabeled monoclonal antibodies for the treatment of malignancy. For more than a decade, radioimmunotherapy has shown great promise for the treatment of B-cell lymphoma. During the past decade, two products targeted to the CD20 antigen on B cells, iodine-131 tositumomab and yttrium-90 (90Y) ibritumomab tiuxetan, have been tested extensively in registration trials for potential licensing approval by the US Food and Drug Administration (FDA). Both products have produced response rates of 70% to 80% in low-grade and follicular lymphoma, and response rates of 50% to 60% in low-grade or follicular lymphoma that has transformed into an intermediate or high-grade lymphoma. Median duration of response to a single course of treatment has been about 1 year, with complete remission rates in one quarter to one third of patients. In February, 2002, 90Y ibritumomab tiuxetan was formally approved by the FDA for the treatment of relapsed or refractory low-grade, follicular, or transformed B-cell lymphoma, including rituximab-refractory transformed lymphoma, thus becoming the first radioimmunotherapeutic agent approved by the agency. The product became available for commercial use in April, 2002. Clinical trials will determine how radioimmunotherapy will be integrated into the treatment of lymphoma.