Immunotherapy approaches have been increasingly used to treat relapsed malignancy and viral infections after solid organ or hematopoietic stem cell transplant. Adoptive cellular immunotherapy with donor T lymphocytes or cytotoxic T cells has proven effective therapy for Epstein-Barr virus lymphoma and some relapsed leukemias after allogeneic stem cell transplantation, and the use of autologous or haploidentical Epstein-Barr virus–specific cytotoxic T lymphocyte lines is being explored after solid organ transplant. More recently, the humanized CD20 antibody rituximab has shown efficacy in the therapy of post-transplant lymphoma. Other antibodies are in trials to treat relapsed malignancy and other viral infections.