A 39-year-old male underwent a nonmyeloablative stem cell transplant (NMAPBPCT) from his HLA-matched sister for recurrent anaplastic large cell lymphoma in CR-2, receiving fludarabine, cyclophosphamide, and rabbit antithymocyte globulin for the preparative therapy. The patient was readmitted on day+33 for persistent culture-negative fevers. He rapidly developed marked elevations of alkaline phosphatase and bilirubin. Liver biopsy showed a periportal infiltrate of large immunoblastic appearing cells. The tumor cells did not stain for CD3/CD20/CD30 and alk protein, but did stain for CD79a/LCA and CD43. In situ hybridization for Epstein-Barr virus (EBV) RNA (EBER 1) was strongly positive in the periportal infiltrating lymphocytes. Fluorescence in situ hybridization (FISH) studies revealed female (XX) cells in the tumor cells and male (XY) in the surrounding hepatic parenchymal cells. The patient developed severe lactic acidosis, oliguric renal failure and expired on day+44. Both donor and patient had positive IgG serologies for EBV VCA and EBNA pretransplant. The donor also had a positive IgM titer for EBV VCA in the pretransplant specimen. The LPD may have been related to the intense immunosuppression of the preparative therapy and the presence of recent EBV infection in the donor.