Results in 164 patients who underwent allogeneic marrow transplantation following busulfan and cyclophosphamide over a 15 year period were analyzed. Age (median 37, range 14-66 years) did not significantly affect the incidence of graft-versus-host disease (GVHD), but patients who received methotrexate with cyclosporine had a significantly lower incidence (P = 0.002) of chronic GVHD compared to those who received methylprednisolone with cyclosporine. Hepatic veno-occlusive disease (VOD) occurred less frequently in chronic phase patients (P = 0.002) and in those transplanted shortly after diagnosis (P = 0.001). Five year leukemia-free survival (LFS) for the entire group was 49% (95% CI 41-57%). For 102 patients who underwent transplantation in chronic phase, results were significantly improved by transplantation at a short interval following diagnosis, particularly within 3 months (P = 0.01), by the use of methotrexate and not corticosteroids for GVHD prevention (P = 0.03), and by use of HLA-identical sibling donors (P = 0.01). Age was not a significant adverse prognostic factor and transplantation was successfully performed in individuals up to age 66. Allogeneic transplantation in CML, including older patients and those with unrelated donors, can be most safely and effectively performed shortly after diagnosis.