Monitoring of cortical blood flow during temporary arterial occlusion in aneurysm surgery by the thermal diffusion method. During aneurysm surgery, regional cortical blood flow (CoBF) was continuously monitored in 12 patients with a thermal diffusion flow probe in an attempt to assess the effects of temporary major arterial occlusion on blood flow and outcome. When the CoBF was above 30 ml/100 g/min, the safe period for temporary clipping applied distal to the perforators was 15 minutes. The occlusion time should be shortened when the CoBF is below 30 ml/100 g/min. Two patients suffered basal infarction, which was not detected by CoBF monitoring. Attention should be paid to the blood flow in the deep structures when a temporary clip is applied at a site proximal to the perforating branches. Direct measurement of CoBF may be of value in estimating the time that temporary occlusion of a major vessel can be tolerated. 