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The jugular bulb oxygen saturation (SjO 2 ) and end-tidal carbon dioxide (ETCO 2 ) were monitored continuously during surgery in six cases of Moyamoya disease who had demonstrated multiple episodes of transient ischemic attacks (TIAs) and/or fluctuating neurological deficits preoperatively. The arterial carbon dioxide tension (PaCO 2 ) levels were also measured repeatedly at predetermined interval. In two cases (group H), the ETCO 2 was controlled at hypercapnic levels during surgery (45.5±1.5 mmHg) and the remaining four (group N) were operated on in a normocapnic state (39.0±2.0 mmHg). The group H patients demonstrated high levels of SjO 2 ranging from 72 to 85%, indicative of excessive hyperemia. One of the group H patients demonstrated mild and transient motor weakness postoperatively. The group N patients demonstrated normal levels of SjO 2 ranging from 66 to 78%. All the patients in both groups demonstrated fluctuations in SjO 2 levels in clear positive correlation with spontaneous changes in PaCO 2 levels. The present findings indicated that: (1) Global carbon dioxide reactivity of cerebral perfusion is well preserved in patients with Moyamoya disease; and (2) hypercapnia in these patients often causes excessive hyperemia. The occurrence of postoperative neurological deficits in association with such an excessive hyperemia suggests that hyperapnia during surgery is not always beneficial. Intraoperative monitoring of SjO 2 is useful for maintaining cerebral perfusion within the optimum range.