A 67-year-old woman underwent a thoracoscopic resection of a large anterior mediastinal cyst. Before surgery, artificial pneumomediastinum was performed with a retrosternal technique. Injection of 400 ml of air from the sternal notch caused emphysema throughout the mediastinum. In those areas, dissection of loose connective tissue was mostly accomplished by the injected air, which formed an air layer around the cyst. On the other hand, emphysema was not apparent in the areas around the left innominate and thymic veins. Artificial pneumomediastinum may be useful as a supplementary technique in a thoracoscopic surgery setting.