Regional cerebral blood flow (rCBF) measurements were performed in three groups of patients before and during 5% carbon dioxide inhalation. In the first group with normal cerebral angiograms, the rCBF increased 60%. Patients with diffuse cerebrovascular disease had baseline flow of 34 ml/100 gm/min with an increase to 44 ml/100 gm/min during hypercapnia. In 20 patients with angiographic evidence of focal vascular disease, blood flow was measured both as the hemispheric mean cerebral blood flow (CBF) and as rCBF in normal or nonfocal regions and in abnormal or focal regions. In nonfocal regions, mean rCBF increased from 37 to 50 ml/100 gm/min; in focal regions, rCBF increased from 28 to 36 ml/100 gm/min. Fifty-seven percent of the focal regions and 71% of the nonfocal regions increased significantly during hypercapnia.