Although no case of lymphoproliferative syndrome occurred among our first 680 transplant patients, 13 cases developed in a subsequent series of 170 patients. This severe condition involved a proliferation of B cells and/or plasma cells that invaded a number of organs, resulting in the deaths of eight patients. Early tapering off of immunosuppressive therapy enabled five patients to recover without loss of the transplant. The factors likely to be involved in the occurrence of malignant lymphoproliferation are immunosuppressive drugs, and introduction of allogeneic EBV-infected cells or reactivation of EBV.