PURPOSE: This study set out to identify the association between the intraperitoneal CO₂ concentrations and postoperative pain by dividing the participants into a control group and 2 experimental groups receiving irrigation (1 L and 2 L), and directly measuring their intraperitoneal CO₂ concentrations with a CO₂ gas detector. METHODS: A total of 101 patients, American Society of Anesthesiologists physical status classification I and II patients aged 18–65 years were enrolled in the study. Group 1 did not receive irrigation with normal saline, while groups 2 and 3 were administered irrigation with 1 L and 2 L of normal saline, respectively, after laparoscopic cholecystectomy. Intraperitoneal CO₂ concentrations were measured with a CO₂ gas detector through the port, and postoperative pain was assessed on a visual analogue scale at 6, 12, and 24 hours after surgery. RESULTS: The intraperitoneal CO₂ concentrations were 1,016.0 ± 960.3 ppm in group 1, 524.5 ± 383.2 ppm in group 2, and 362.2 ± 293.6 ppm in group 3, showing significantly lower concentrations in groups 2 and 3. Postoperative pain was significantly lower in group 3 at 6 hours after surgery, and in groups 2 and 3 at 12 hours after the surgery. However, there was no significant difference between the 3 groups in postoperative pain 24 hours after the surgery. CONCLUSION: This study found a causal relationship between the amount of normal saline used for irrigation and the intraperitoneal CO₂ concentrations in that irrigation with normal saline reduces pain on the day of the surgery.