The effect of CO2 on cerebral blood flow was studied in 10 patients with unilateral internal carotid obstruction. Inhalation of 5% CO2 increased the cerebral blood flow significantly from 28.8 to 40.5 ml/100 gm of brain weight 1 min (a 40% increase). A significant correlation was established between cerebral blood flow, neurological defects, and angiographic crossfilling. Increased cerebrovascular resistance in carotid obstruction is attributed to increase cerebrovascular tonus, which tends to preserve pressure gradients all along the cerebral arterial tree in the presence of a decreased volume of inflow. CO2 inhalation increased the cerebral blood flow by virtue of reduction of this cerebrovascular resistance, thereby serving to reduce stasis in less efficiently perfused arteries. Intermittent inhalation of 5% CO2 in air is recommended as an adjunct to vascular surgery in treatment of unilateral carotid obstructions.