BACKGROUND: The purpose of this study was to evaluate the effects of lung disease on the difference between arterial and end-tidal carbon dioxide tension by placing patients from the supine to the lateral decubitus position and by the changes from two lung ventilation (TLV) to one lung ventilation (OLV) during thoracic surgery. METHOD: Fifteen patients who had no parenchymal lung disease were selected for control group and 15 patients who had parenchymal lung disease on non-dependent lung were selected for disease group. All patients had been intubated with double lumen endobronchial tubes and respiration was controlled with a rate of 14-15 breaths per minute and tidal volume 8 ml/kg. End-tidal and arterial carbon dioxide tension were measured at three different measurement periods (supine plus TLV, lateral decubitus plus TLV, lateral decubitus plus OLV). RESULTS: The arterial to end-tidal carbon dioxide tension difference was more increased in disease group than control group. But there was no significant difference in arterial to end-tidal carbon dioxide tension with position change, ventilation method change in each groups. CONCLUSION: We conclude that the arterial to end-tidal carbon dioxide tension difference is increased in lung disease, but it does not changed with position and ventilation method change.