Bronchiolitis obliterans syndrome is a potentially fatal complication of allogeneic hematopoietic cell transplantation that is defined as the development of new fixed airflow obstruction within the first 2 years after transplantation. The prevalence of this syndrome is estimated to be 5.5% among all allogeneic hematopoietic cell transplantation recipients, and 14% among all long-term survivors who develop chronic graft-versus-host disease. Although little is known about the pathogenesis of this syndrome, there are now clear guidelines for clinical diagnosis of bronchiolitis obliterans syndrome, and there appear to be promising new approaches towards treatment that can significantly reduce the morbidity associated with corticosteroid therapy. Future research efforts should target the identification of clinical biomarkers for risk stratification, early diagnosis and treatment-response assessment.