Laparoscopy can be performed while patients are under total intravenous anesthesia (TIVA), or sedated and breathing spontaneously through the normal airway. Respiratory monitoring is difficult when patients are sedated or anesthetized, however. The purposes of this study were to evaluate the reliability of end-tidal carbon dioxide (ETCO2) measurement for monitoring arterial carbon dioxide pressure (PaCO2), and to assess the PaCO2/ETCO2 gradient among patients receiving TIVA while breathing spontaneously through the normal airway. Sixty patients were divided into two groups: group 1 patients (n = 30) received general anesthesia with controlled ventilation, while group 2 patients (n = 30) received TIVA with spontaneous breathing through the normal airway; ETCO2 was sampled through a 10-French suction catheter inserted into the nasopharynx via the nasal airway. Arterial blood gas and ETCO2 were recorded at the time of preinduction, induction, CO2 insufflation, and change to Trendelenburg tilt position (20 degrees-30 degrees), and at 10-minute intervals thereafter. The results showed that ETCO2 was highly correlated with PaCO2 in group 1 (correlation coefficient r = 0.85), but not in group 2 (r = 0.55). In group 2, the PaCO2/ETCO2 gradient increased as time elapsed, with significant differences (p < 0.05) between the values at induction and those at 30 minutes after the change to the Trendelenburg position and thereafter. These results indicate that the ETCO2 and PaCO2 values correlate well during the first 20 minutes after the change to the Trendelenburg position in laparoscopy patients receiving TIVA with spontaneous breathing, but that PaCO2 monitoring is still necessary.