Posttransplant lymphoproliferative disorders (PTLD) are EBV-associated lymphoid neoplasms that are caused by the uncontrolled growth of EBV-infected B lymphocytes. The clinical presentation of PTLD can range from benign polygonal lymphoproliferative disorders to aggressive monoclonal immunoblastic lymphomas. In this report, we describe a seronegative lung transplant recipient who developed an immunoblastic lymphoma 4 months after lung transplantation from a seropositive donor. The neoplastic cells expressed B lymphocyte markers (CD19+, CD20+, sIgM+, kappa+) as well as the EBV antigen EBNA-2. A cell line with similar cytologic features spontaneously grew from in vitro cultures of the patient's peripheral blood mononuclear cells. The cell line and the lymphoma were EBV+, expressed a similar spectrum of B cell surface proteins, and had the donor's HLA haplotype. Analysis of immunoglobulin gene rearrangements and viral terminal repeat sequences revealed that the cell line and the tumor represented distinct B cell clones. Cultured peripheral blood mononuclear cells were restimulated in vitro with the EBV transformed cell line and tested for cytolytic activity. The host T cells demonstrated high levels of cytolytic activity against the tumor cell line that was abrogated by the addition of a anti-monomorphic HLA class I monoclonal antibody (mAb) (W6/32). These studies indicate that cells of donor origin can persist in the transplanted organ and may lead to an EBV-associated posttransplant lymphoma.