The bilateral use of bispectral index (BIS) monitoring in a 68-year-old male who underwent right carotid endarterectomy (CEA) under general anesthesia (GA) is described. During three episodes of right internal carotid artery cross-clamping intraoperatively, the right BIS value rose relative to the left within 1 min of clamping, followed by a return to baseline within 1-2 min of clamp release. Since unilateral BIS values can decrease or increase in response to cross-clamping, a significant difference in left and right BIS values may provide a simple and easily available method to detect cerebral hypoperfusion during CEA under GA.