Four cases of patients whose postpneumonectomy spaces had been controlled with SF6 were examined at autopsy, which showed that the spaces were entirely covered with thickening parietal pleura and that the mediastinum had moderate elasticity. Histologically, fibrotic changes with minimum inflammation patterns were seen in the pleura. We consider that mediastinal elasticity becomes a factor in improving pulmonary function and the above observations confirmed that SF6 prevents the proliferative reaction of postpneumonectomy spaces without any toxicity.