AbstractLymphomas 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‐B