The purpose of this study is to evaulate the relationship between end-tidal PCO2(PetCO2) and arterial PCO2(PaCO2) by placing patients from the supine to the lateraI decubitus position and by the changes from two lung ventilation(TLV) to one lung ventilation(OLV) during noncardiac thoracic surgery in 20 patients. All patients had been intubated with double-lumen endobronchial tubes and respirations were controlled with a rate of 14- l7 breaths per minute and tidal volume of 8 ml/kg by an anesthetic ventilator. End tidal PCO2 and arterial PCO2 were measured at three different measurement points(supine plus TLV, lateral decubitus plus TLV, and lateral decubitus plus OLV). End tidal PCO was obtained by mainstream infrared analysis, and arterial blood samples for arterial PCO2 were taken from the radial artery simultaneously. The results were as follows: l) The mean difference between arterial and end tidal carbon dioxide tension(Pa-etCO2) was 7.5±2.9mmHg at the supine position, TLV(r=0.76, P<0.01). 2) The mean Pa-etCO2 was 6.2±3.1 mmHg at the lateral decubitus position, TLV(r =0.68, P < O.ol ). 3) The mean Pa-etCO2 was 7.1±2.5 mmHg at the lateral decubitus position, OLV(r=0.85, P< 0.01). 4) The changes in Pa-etCO2 were very slight during thoracotomy and endobronchial anesthesia, these were of negligible clinical importance. We conclude that measurement of PetCO2 is reliable as a guideline of ventilation during endobronchial anesthesia.