A total of 109 patients (aged 6-66 years; median, 46 years) with myelodysplastic syndrome (MDS) were treated with busulfan (BU) targeted to plasma concentrations of 800 to 900 ng/mL plus cyclophosphamide (CY), 2 x 60 mg/kg, and hemopoietic stem cell (HSC) transplantation from related (n = 45) or unrelated donors (n = 64). At the time of transplantation, 69 patients had less than 5% myeloblasts in the marrow, and 40 patients had more advanced disease. All but 2 evaluable patients had engraftment. The Kaplan-Meier estimates of 3-year relapse-free survival (RFS) were 56% for related and 59% for unrelated recipients. The cumulative incidences of relapse were 16% for related and 11% for unrelated recipients. Nonrelapse mortality (NRM) at 100 days (3 years) was 12% (28%) for related and 13% (30%) for unrelated recipients. The only factor significant for RFS was the etiology of MDS (de novo better than treatment related; P =.03). Factors significantly correlated with relapse were advanced French-American-British classification (P =.002) and International Prognostic Scoring System score (P =.009), poor-risk cytogenetics (P =.03), and treatment-related etiology (P =.03). None of the factors examined was statistically significant for NRM. Patient age and donor type had no significant impact on outcome. RFS tended to be superior in patients receiving transplants with peripheral blood rather than marrow stem cells. Thus, a targeted BUCY regimen provided effective transplant conditioning for patients with MDS receiving transplants from HLA-identical siblings or alternative donors. Although there was still considerable nonrelapse morbidity and mortality, the present regimen was used successfully even in patients older than 60 years of age.