Background The occurrence of bronchiolitis obliterans syndrome (BOS) after allogeneic hematopoietic stem cell transplantation (alIo-HSCT) is rare but severe.We examine the role of pre-HSCT chemotherapeutic exposure,pre-HSCT comorbidities,and transplant-related complications in the development of BOS after allo-HSCT.Methods A nested case-control study was designed.Cases with BOS and controls matched for the year of alIo-HSCT and length of the follow-up were identified from a cohort of 1646 patients who underwent alIo-HSCT for treatment of hematologic malignancies between 2006 and 2011.Antithymocyte globulin was used in the partial matched related and unrelated matched donor HSCT,or patients with severe aplastic anemia.Results Thirty-six patients suffered from BOS; the mean age at the time of presentation was (32.7±12.4) years,and the mean time to presentation was (474±350) days post-HSCT.A pre-HSCT cyclophosphamide dose of ＞3.2 g/m2 (OR=8.74,P=0.025),chronic graft-versus-host disease (moderate to severe) (OR=12.02,P=0.000),and conditioning regimens without antithymocyte globulin (OR=2.79,P=0.031) were independently associated with BOS.Conclusions We found that higher pre-HSCT cyclophosphamide exposure,a conditioning regimen without antithymocyte globulin,and moderate to severe chronic graft-versus-host disease are significantly and independently associated with BOS.Based on these results,we can identify patients who are at a higher risk of developing BOS after alIo-HSCT,select a more appropriate therapeutic strategy,and improve the outcome of HSCT recipients.