The cerebral oxygenation state of five patients was continuously and non-invasively monitored using near-infrared spectroscopy (NIRS) during carotid artery balloon occlusion for comparison with somatosensory evoked potential (SEP) monitoring and regional cerebral blood flow (rCBF) measurements. In patients with marked reduction of rCBF and deteriorated SEP, NIRS demonstrated a continuous decrease in oxy-hemoglobin and total hemoglobin and an increase in deoxy-hemoglobin, indicating the occurrence of severe hypoxia in the ipsilateral hemisphere because of inadequate collateral circulation. In contrast, NIRS showed only a transient (< 2 min) change in these parameters in the patients who showed no remarkable changes in SEP or rCBF, suggesting that well-developed collateral circulation immediately improved cerebral hemodynamics in the ipsilateral hemisphere. These results suggest that NIRS can provide direct and valuable information on cerebral hemodynamic changes during carotid artery balloon occlusion.