Over a period of 8.5 years (February 1988 to October 1996), 1423 patients with chronic myelogenous leukemia (CML) underwent unrelated donor (URD) bone marrow transplants (BMTs) facilitated by the National Marrow Donor Program (NMDP) at 85 transplant centers. One hundred thirty-seven evaluable (9.9%) patients failed to engraft, and an additional 83 (6.6%) evaluable patients experienced late graft failure. Grade III/IV acute graft-versus-host disease (GVHD) developed in 33% of patients (95% confidence interval [CI], 30%-36%). The incidence of extensive chronic GVHD was 60% (95% CI, 56%-63%) at 2 years. Only 5.7% of patients (95% CI, 3.6%-7.8%) transplanted in chronic phase developed hematologic relapse at 3 years. Several factors were independently associated with improved disease-free survival (DFS), including transplant in chronic phase, transplant within 1 year of diagnosis, younger recipient age, a cytomegalovirus seronegative recipient, and development of no or mild acute GVHD. The combined effect of these factors on outcome is manifest in a subset (n = 157) of young (less than 35 years), chronic phase patients transplanted within 1 year of diagnosis using HLA-matched donors who had 63% (95% CI, 53%-73%) DFS at 3 years. URD BMT therapy for CML is both feasible and effective with more frequent and more rapid identification of suitable donors. Early URD transplant during chronic phase yields good results and should be considered in CML patients otherwise eligible for transplant but without a suitable related donor.