Initial treatment for posttransplant lymphoproliferative disorder (PTLD) usually involves discontinuation of immunosuppressants. Anti-CD20 monoclonal antibody and/or antivirals are typically employed for persistent disease. Chemotherapy is generally reserved as a final option. The role of radiation therapy, and doses required, have not been well established. Low-dose involved field radiation therapy was used in three pediatric bone marrow transplant (BMT) patients with biopsy-proven PTLD. One patient received a matched T-cell-depleted BMT for dyskeratosis congenita. Two patients with acute myelogenous leukemia received an unrelated umbilical cord blood transplant, and a matched-related allogeneic BMT. All patients required intubation for respiratory distress due to PTLD. Initial treatment was discontinuation or decrease in FK-506. Anti-CD20 antibody was started in all patients, and foscarnet in one patient. All patients were treated with three 150-cGy fractions, for a total dose of 450 cGy. Time from BMT to development of PTLD was 4, 2, and 32 months, respectively. Duration of observation on initial medical therapy prior to irradiation was 11 days, 12 days, and 1 day, respectively. The radiation was well tolerated with no acute complications. Two patients were extubated at 8 and 4 days postradiation, respectively. The first patient had complete radiographic resolution of the mass and adenopathy at 4 months after radiation. The second died of pulmonary hemorrhage and disseminated aspergillosis infection, but had significant regression of disease in the irradiated area 15 days after radiation. The third had pronounced shrinkage of his mediastinal mass. A biopsy was taken of a persistent mass 4 months after radiation, with no evidence of lymphoproliferative disease. These cases demonstrate that low-dose radiation for PTLD is effective for palliation and produces a durable response with no acute toxicity.