A 24-year-old woman was admitted to our hospital due to moderate asthmatic attacks. Dyspnea and hypoxemia progressed gradually despite medication. A chest roentgenogram revealed left unilateraly hyperlucency with pneumomediastiumn and subcutaneous emphysema. Swyer-James syndrome was diagnosed. Several cases of Swyer-James syndrome with bronchial asthma airway hyperresponsiveness have been reported, but we know of no reports of Swyer-James syndrome with pneumomediastinum and subcutaneous emphysema due to prolonged asthmatic attacks. Pneumomediastinum and subcutaneous emphysema may be caused by abnormally high pressures in the bronchial lumen and alveolar space during asthmatic attacks, because the emphysematous lesion may be structurally weak.