The effects of intraperitoneal CO2 insufflation on middle cerebral arterial blood flow velocity were evaluated in 10 patients undergoing laparoscopic cholecystectomy under general anesthesia with nitrous oxide, oxygen, and isoflurane. Blood flow velocity was measured using transcranial Doppler ultrasonography. During CO2 insufflation, Paco2 and the end-tidal CO2 concentration (PETCO2) increased significantly compared with the preinsufflation baseline value (P < 0.01) while ventilation was kept constant. Cerebral blood flow velocity also increased significantly in comparison with the baseline value (P < 0.01). These values still exceeded baseline values 10 min after deflation of the peritoneal cavity. A significant positive correlation was observed between blood flow velocity and Paco2 (P < 0.001). Our results suggest that intraperitoneal CO2 insufflation during laparoscopic cholecystectomy increases cerebral blood flow and that this is probably due to an increased Paco2.