Bronchiolitis obliterans (BO) is the only late-onset pulmonary complication after allogeneic hematopoietic stem cell transplantation (HSCT) to be recognized as a pulmonary graft-versus-host disease, affecting approximately 10% of HSCT recipients. Although the diagnosis of BO used to require a lung biopsy, it is now based primarily on pulmonary function tests and is called bronchiolitis obliterans syndrome (BOS). However, there are subtypes of BOS with different respiratory function profiles. Similarly, BOS may correspond to different histological subtypes. Unfortunately, there is no curative treatment for BOS. In addition to a better understanding of the pathophysiology of BOS, current research aims to identify biomarkers capable of diagnosing BOS at an early stage where treatment would be more effective.