Patients immunosuppressed after organ transplantation have an increased frequency of lymphoproliferative disorders, known as posttransplantation lymphoproliferative disorders (PTLDs). In recipients of bone marrow allografts, PTLDs are often of donor origin. In only a few cases of lymphoma arising in solid-organ transplant recipients has the origin from host or donor lymphocytes been established. The authors have analyzed 11 cases of PTLD from Massachusetts General Hospital, arising in seven male and four female patients, aged 8 to 63, five with renal, four with cardiac, and two with hepatic allografts. Using the polymerase chain reaction (PCR) to investigate genetic polymorphism at the D4S174 locus on chromosome 4, the Rb1.20 locus on chromosome 13, and the D19S178 locus on chromosome 19, only one tumor (previously reported) was of donor origin, whereas 10 were of host origin. Follow-up revealed that six patients died of PTLD, one was alive with recurrent PTLD, and four were alive and well or had died of other causes, including the patient with donor-origin PTLD. Based on these cases and on a review of previously reported cases, the authors conclude that the majority of PTLDs in solid organ recipients are of host origin. There appears to be a trend toward a greater likelihood of persistent or recurrent PTLD among solid organ recipients with host-origin tumors than among those with donor-origin tumor.