Apart from various infective complications in early stage post-HSCT, bronchiolitis obliterans syndrome (BOS) has been a well-known late complication that can result in high mortality. It has been mostly associated with those who develop chronic graft-versus-host disease after allogeneic HSCT. Clinically, the diagnosis of BOS is largely based on demonstration of obstructive lung function abnormalities and air-trapping in computed tomography scan of thorax. Pathologically, bronchiolitis obliterans is characterized by both inflammatory and fibrotic reactions in the small bronchioles leading to subsequent obliteration. Upon diagnosis of BOS post-HSCT, inhaled corticosteroid (with or without bronchodilators) is commonly prescribed as anti-inflammatory agent, though with undocumented clinical efficacy. Unfortunately, there is still a lack of reliable biomarkers that can predict or allow early detection of BOS, preferably in the early and potentially reversible stage of development of BOS.