Therefore there is intense interest in the search for biomarkers that can help to predict the occurrence of BOS after HSCT. Apart from biomarkers (e.g., cytokines) in blood, there has been recent development in the sampling of airway lining fluid by a non-invasive method, i.e., collection of exhaled breath condensate (EBC). In airway diseases such as asthma or chronic obstructive pulmonary disease, EBC has been found to have various cytokines which can serve as potential biomarkers of disease activity. Since BOS is largely a small airway disease, it becomes logical to investigate the profile of biomarkers in EBC as predictors for BOS after HSCT.