Postpneumonectomy syndrome is a rare complication of pneumonectomy. We experienced a case of acute respiratory failure resulting from postpneumonectomy syndrome that was dramatically improved by intrathoracic gas injection therapy. A 52-year-old man who underwent left pneumonectomy as a result of lung adenocarcinoma (cT2aN1M0, stage IIA) about 1 year previously developed severe respiratory distress and was taken to our hospital by ambulance. His right lung was markedly expanded, and the mediastinum was significantly displaced to the left side on a chest X-ray. His trachea and bronchus were extended and compressed by the aorta and vertebral body on the CT scan. We determined that these constructive changes had caused his respiratory disturbances. Immediately after the injection of air into his left thoracic space through a chest drainage tube, his mediastinal position was corrected, and his respiratory distress dramatically improved. After 1 month, we injected C₃F₈ gas instead of air to maintain this effect. He currently receives this treatment once every 3 months and has maintained a good clinical condition. Although several issues regarding intrathoracic gas injection therapy must be kept in mind, this procedure can be considered minimally invasive and clinically useful.