Cortical blood flow was evaluated at the time of surgery with a thermal diffusion flow probe in 25 extracranial-intracranial bypass operations. The procedures were performed 14 times for carotid occlusion, 6 times for inaccessible carotid stenosis, on 3 occasions for middle cerebral occlusion, once for basilar occlusion and once for middle cerebral stenosis. Fifty-seven different cortical regions were evaluated before and after completion of the anastomosis. During temporary occlusion of the cortical vessel, no significant changes in cortical blood flow were identified. The mean flow pre-bypass was 58 ml/100 g/min and postbypass was 66. This was a significant difference. When initial ischemic areas were evaluated, flows increased from 36 ml/100 g/min to 52 ml/100 g/min. These postbypass flows were statistically different than the flow values in the nonischemic areas of 75 ml/100 g/min. Blood pressure and arterial carbon dioxide tension did not vary significantly. Extracranial-intracranial bypass increases cortical flow in ischemic areas although this does not bring the flow into the normal range.