Regional cortical blood flow was monitored at craniotomy in 32 patients with a thermal diffusion flow probe in an attempt to assess the effects of surgical conditions on flow. Changes in flow due to vascular manipulation, retraction, hypotension, and hyperventilation were readily apparent. Eleven patients with supratentorial tumors had intermittent measurements of cerebral blood flow; the expected differences in blood flow with halothane anesthesia and hyperventilation were observed. Fourteen patients with aneurysms were monitored continuously and demonstrated a reduction in flow during hypotension; in two of these cases, the reduction was to disastrously low levels. Flow reduction occurred secondary to branch occlusion by the aneurysm clip in a patient with three middle cerebral artery aneurysms and was corrected by changing the clip position. Direct measurement of regional flow may be of value in estimating the safety of temporary clips and the degree of hypotension that can be tolerated.