Apart from measuring circulating biomarkers in blood, exhaled breath condensate (EBC) has recently emerged as a non-invasive sampling method for real-time analysis and evaluation of oxidative stress biomarkers in the lower respiratory tract airways, especially in various lung diseases including asthma, chronic obstructive pulmonary disease, and lung cancer. As bronchiolitis obliterans is predominantly a disease of the small bronchioles, it is highly likely to be associated with changes in various inflammatory and oxidative stress biomarkers in EBC. However, the role of measuring EBC biomarkers in predicting the occurrence of BOS after HSCT has not been studied. Therefore, the current study aims to evaluate the temporal changes of various oxidative stress biomarkers and cytokines in EBC and blood in patients with haematological conditions who undergo allogeneic HSCT, with regard to the subsequent development of BOS.