Impairment of pulmonary function is reported in around 60% of children after allogeneic hematopoietic stem cell transplantation (HSCT), used as a treatment of high-risk acute leukemias and severe hematological disorders.1 The decline in pulmonary function is transient in most patients, but 5%–10% of children undergoing HSCT develop bronchiolitis obliterans (BO), which is a severe manifestation of chronic graft-versus-host disease (cGvHD). This progressive obstructive airway disease has a 5-year mortality rate of 33%–55% and may lead to permanent severe disability in a large proportion of those who survive.2 While early diagnosis and immunosuppressive treatment of BO may improve the prognosis, initial symptoms are often mild, and regular monitoring with spirometry is therefore important in the follow-up after HSCT. Importantly, however, young children below 5 years of age cannot cooperate to spirometry and reliable measurements may be hard to obtain even in older patients. Therefore, early predictive biomarkers of pulmonary GvHD are highly needed to guide clinical decisions.