Recent clinical, pathological, and molecular studies have increased our understanding of posttransplantation lymphoproliferative disorders (PT-LPDs). Studies have shown that the majority of PT-LPDs arising in bone marrow transplant recipients are of donor origin; however, the source (host or donor) of the lymphoid cells that make up PT-LPDs arising in solid organ transplant recipients has not been systemically investigated. In this study, 18 PT-LPDs occurring in 16 organ transplant recipients (13 heart, 2 kidney, 1 lung), 9 donor tissues (for 10 recipients), and 14 uninvolved recipient tissues (from 12 patients) were examined employing restriction fragment length polymorphism analysis to determine their host or donor origin. The PstI-digested DNAs were analyzed by Southern blot hybridization using two highly informative polymorphic probes that map to chromosome 21 (CRI-PAT-pL427-4) and chromosome 7 (CRI-PAT-pS194). All solid organ PT-LPDs with corresponding uninvolved recipient DNA showed identical hybridization patterns; none of the PT-LPDs exhibited a hybridization pattern that matched donor DNA. These findings suggest that the vast majority of PT-LPDs arising in solid organ transplant recipients, in contrast to those arising in bone marrow transplant recipients, are of recipient origin.