Objective To determine whether bronchiolitis obliterans syndrome (BOS) after bone marrow transplantation can be predicted in light of air trapping. Methods Thirty three cases of BOS (BOS group) and 111 normal patients (normal group) were comparatively reviewed for the CT characteristics (including frequency, dynamic change and diagnostic potency) of air trapping. The analysis was mainly based on the last CT scans before occurrence of BOS. Results The frequency of air trapping, especially mosaic air trapping or extensive air trapping, was higher in the BOS group than in the normal group (P=0.03). The median total air trapping score was higher in the BOS group than in the normal group (P=0.01). The increase of air trapping extent with the progress of disease was more common in the BOS group (50.0%, 9/18), whereas a decrease of air trapping extent, disappearance or intermittent appearance of air trapping was more common in the normal group (60.0%, 15/25) during the follow up period. When mosaic air trapping or extensive air trapping was used as the diagnostic threshold, the specificity was high (90.5%, 96.7%), but the sensitivity was low (29.4%, 15.8%). Conclusion Occurrence of air trapping is a clue for development of BOS in patients receiving bone marrow transplantation. Mosaic air trapping and extensive air trapping are insensitive but specific CT findings.