Posttransplant lymphoproliferative disorder (PTLD) is a common and life-threatening complication of immunosuppression used to prevent rejection of solid organ and bone marrow transplants. There is no standardized treatment algorithm, but numerous management strategies are available. We describe a patient who developed a solitary lymphoproliferative lesion in the porta hepatis 9 months after orthotopic liver transplant. Following reduction in immunosuppression with no response, she was treated with involved field radiotherapy utilizing CT-based treatment planning. A partial radiographic response was obtained, and she has not developed disease in the engrafted liver or systemically. Based on the present case report, involved field radiotherapy seems to be a reasonable treatment option for patients with localized PTLD. Int. J. Cancer (Radiat. Oncol. Invest.) 90:104-109, 2000. Copyright 2000 Wiley-Liss, Inc.