In five female bone marrow transplant (BMT) recipients of sex-mismatched donor marrow, Y-chromosome specific in situ hybridization was performed on formalin-fixed, paraffin-embedded sections of the medulla to detect the male donor marrow-derived cells. Y-chromosome-bearing cells (Y-cells), thereby donor-derived, were matched with leukocyte common antigen (LCA)-reactive cells in adjacent sections immunostained with anti-LCA antibody. Y-cells included mononuclear leukocytes (MNL) within the vessel lumen and infiltrating the perivascular space and parenchyma, and “perivascular cells.” We have, therefore, concluded that donor marrow-derived MNL, though limited in number, do enter the normal-appearing brain and can transform to “perivascular cells” in human BMT recipients. It remains, however, to be confirmed whether MNL entering the normal adult CNS parenchyma transform to ramified microglia.