The safety and efficacy of a 4-day myeloablative conditioning (MAC) regimen consisting of Bu 3.2 mg/kg and fludarabine 40 mg/m(2)/day for HLA-identical sibling allogeneic hematopoietic cell transplantation (HCT) in myeloid malignancies was investigated in 133 patients (median age, 47 years; range 19-74 years) with de novo AML (60%), secondary AML (20%) or myelodysplastic syndrome (20%). All patients engrafted. Hepatic veno-occlusive disease occurred in five patients (4%), and severe toxicities, mostly mucositis, occurred in twenty-three (17%) patients. The non-relapse mortality (NRM) at 100 days was 1.5%. The incidences of acute GVHD grade 2-4 and grade 3-4 were 32 and 13%, respectively. At a median follow-up of 38 months, the cumulative incidence of chronic GVHD was 67%. The relapse incidence was 30% (27 and 31%, respectively, in patients with early- and late-stage disease), and the overall NRM was 15%. The actuarial 4-year disease-free survival (DFS) and overall survival (OS) were 54 and 62%, respectively. Patients aged <50 years had better outcomes compared with older patients (DFS 64 vs 42%, P=0.006; OS 73 vs 47%, P<0.001, respectively).