BACKGROUND: It has been known that arterial carbon dioxide tension is 4~5 mmHg higher than end-tidal carbon dioxide tension in healthy adults during general anesthesia. But negative arterial to end-tidal PCO2 difference was reported in pregnant patients undergoing cesarean section. The purpose of this study was to elucidate the difference between arterial and end-tidal PCO2 in anesthetized patients with reduced functional residual capacity. METHODS: 90 patients were divided into 3 groups i.e. control group (n=30), obese group (n=20, body weight more than 20% greater than ideal weight), pregnant group (n=40). All patients had no cardiac or respiratory abnormalities and never smoked. Arterial blood gas analysis and measurement of end-tidal PCO2 were done 20 minutes after induction of anesthesia in control and obese group and just before uterine incision and 20 minutes after fetal delivery in pregnant group. RESULTS: There were significant correlations between arterial and end-tidal PCO2 in all groups. The incidences of negative arterial to end-tidal PCO2 difference were 10% in control group, 40% in obese group, 42.5% in pregnant group (p<0.05). CONCLUSION: From this study, it is concluded that patients with reduced functional residual capacity have more incidences of negativity than normal patients in the values of arterial to end-tidal PCO2 difference during general anesthesia. So when the tight control of PaCO2 is required in patients with reduced FRC, we recommend to measure PaCO2 for better anesthetic management.