Lymphomas are an uncommon complication of solid organ transplantation and rarely occur after marrow transplantation. When post-marrow transplant lymphomas have occurred, they have been of donor cell origin and when sought, Epstein-Barr virus DNA has been found in the tumor. A 21-year-old woman developed a poorly differentiated lymphocytic lymphoma 6 months after bone marrow transplantation for acute myeloid leukemia in remission. Cyclosporin A had been used as an immunosuppressant. A chromosomal polymorphism demonstrated that the tumor was of host origin and contained a monoclonal tumor marker, 46,XX INV 4 (p16q12). The tumor did not contain the DNA of the Epstein-Barr virus.