We reported 5 patients who developed air-leak syndrome (ALS) including pneumothorax, pneumomediastinum and subcutaneous emphysema after allogeneic stem cell transplantation (SCT). The underlying diseases were AML (n=2), ALL (n=1), MDS (n=1), and CML (n=1). All patients received allogeneic SCT from related donors including 2 donors with HLA mismatch. Total body irradiation was performed as a conditioning regimen in all patients. Late-onset noninfectious pulmonary complications (LONIPC) were detected in all patients before the development of ALS. The interval from diagnosis of LONIPC to onset of ALS was 10-360 days (median, 20 days). Four of 5 patients were treated with corticosteroid for chronic graft-versus-host disease and/or LONIPC. To date, three patients have died of respiratory failure. The others are currently alive and one of these surviving patients is receiving home oxygen treatment. Physicians should be aware of this rare complication following LONIPC, because treatment of ALS is difficult in some patients.