Secondary T-cell lymphoproliferation after marrow transplantation. Secondary lymphoproliferative syndromes in immunosuppressed patients have been characterized as polyclonal or monoclonal B-lineage disorders nearly always associated with Epstein-Barr virus (EBV) infection. The authors now report three patients with a distinctly different lymphoproliferative syndrome. Two patients with common acute lymphoblastic leukemia antigen (CALLA) (CD10)-positive acute lymphoblastic leukemia and one patient with acute myelogenous leukemia, respectively, received high-dose chemoradiotherapy followed by marrow transplantation from either an HLA-identical sibling or HLA-mismatched parent. All three patients developed severe graft-versus-host disease (GVHD), requiring immunosuppressive treatment with corticosteroids. A secondary malignant T-cell lymphoproliferation occurred 2, 21, and 43 months, respectively, after marrow transplantation. In all three cases the lymphoid cells expressed T-cell surface antigens and were morphologically and immunophenotypically distinct from the malignant cells present before transplantation. One tumor was of host cell origin, one was probably of donor origin, and the tumor origin in the third case could not be determined. The authors were unable to find any evidence for EBV, human T-cell lymphotropic virus type I or II, human immunodeficiency virus, or human herpesvirus 6. 