Peripheral blood cultures of hemopoietic precursor cells from 30 patients with chronic myelogenous leukemia showed different growth patterns before bone marrow transplantation. A strong increase of free and clustered macrophages was seen in 11/30. Of these 11 patients, 10 developed Graft-versus-Host Disease (GvHD). Of 19 patients without the macrophage pattern, 4 developed GvHD (p = 0.004). Of 14 patients with GvHD, 10 had shown the macrophage pattern before bone marrow transplantation, compared to 1/19 without GvHD (p = 0.004). We postulate that excess macrophages in the bone marrow recipient trigger GvHD by enhancing presentation of recipient antigens to donor T-lymphocytes, and that their presence is predictive of GvHD in CML.