Allogeneic hematopoietic stem cell transplantation is the only curative therapy for myelodysplasia (MDS). To identify factors influencing transplantation outcome, we studied 452 recipients of HLA-identical sibling transplants for MDS from 1989 to 1997, reported to the International Bone Marrow Transplant Registry. Patients with treatment-related MDS or unclassified MDS were excluded. Median age was 38 years (range, 2-64 years). Sixty percent had refractory anemia with excess blasts (n = 136) or with excess blasts in transformation (n = 136). Conditioning regimens included total body irradiation in 199 (44%) cases. Marrow was T-cell depleted for 58 (13%) transplants. Cumulative incidences of neutrophil engraftment, grades II-IV acute graft-versus-host disease (GVHD), and chronic GVHD were 91% (95% confidence interval [CI], 88%-93%), 36% (95% CI, 31%-40%), and 39% (95% CI, 33%-44%), respectively. Three-year transplantation-related mortality (TRM), relapse, disease-free survival, and overall survival rates were 37% (95% CI, 32%-42%), 23% (95% CI, 19%-27%), 40% (95% CI, 36%-45%), and 42% (95% CI, 37%-47%), respectively. Multivariate analyses showed that young age and platelet counts higher than 100 x 10(9)/L at transplantation were associated with lower TRM and higher disease-free and overall survival rates. Relapse incidence was higher in patients with high percentages of blasts in the marrow at transplantation or presentation, with high International Prognostic Scoring System scores at diagnosis, and with T-cell-depleted transplants. These findings indicate that transplantation from an HLA-identical sibling offers the possibility of long-term, disease-free survival to patients with MDS. Best candidates are younger patients with a low percentage of blasts and preserved platelet counts.