STUDY OBJECTIVE: To determine the efficacy of allogeneic bone marrow transplantation for severe myelodysplasia, and to identify variables predictive of outcome. DESIGN: Case series study. SETTING: A referral-based bone marrow transplant center. PATIENTS: Consecutive series of 59 patients with myelodysplasia or closely related disorders and either life-threatening cytopenia or a progressive increase in marrow blast percentage. INTERVENTION: Patients were treated with high-dose cyclophosphamide and total body irradiation followed by allogeneic bone marrow transplantation from either an HLA-identical (n = 45) or HLA-partially matched (n = 14) donor. MEASUREMENTS AND MAIN RESULTS: The product-limit estimate for disease-free survival 3 years after transplant is 45% (95% CI, 32% to 59%). The commonest causes of death after transplant were disease recurrence, interstitial pneumonia, and graft-versus-host disease, accounting for eight deaths each. In a univariate analysis, younger patients, those with shorter disease duration, and those whose disease was characterized by an abnormal cytogenetic karyotype had better survival and disease-free survival than the group as a whole. In a multivariate analysis, younger age and abnormal karyotype were independent predictors of improved disease-free survival and overall survival. Patients who received transplants when they had fewer blasts in their bone marrow had a decreased chance for disease recurrence when compared with patients with excess blasts. CONCLUSIONS: Bone marrow transplantation offers a potential cure for many patients with myelodysplasia. Best results can be expected in younger patients who receive transplants relatively early in their disease course.