Effects of hypocapnia on cerebral oxygen consumption (CMRO2) and blood flow (CBF) in cerebral ischemia were studied in 19 patients. The CMRO₂ did not change significantly during hypocapnia within the whole group of patients, because 10 out of 19 cases showed a decrease (p < 0.001) and other 9 showed an increase (p < 0.01) of CMRO₂ during hypocapnia. The first 10 showed higher resting CMRO₂ (p < 0.001) and arteriovenous differences of oxygen content (AVDO₂; p < 0.02) than the other 9 However, the resting CBF and CO₂ reactivity to hypocapnia were not different between them, and clinical situations were also similar. A dissociation between flow and metabolism was suggested in the first 10 with rather preserved CMRO₂, while reduced metabolic demands were suggested in the other 9. Different responses of CMRO₂ to hypocapnia are expected in cerebral ischemia, i.e. in cases with rather preserved CMRO₂ it decreases despite an AVDO₂ increase, suggesting a capability of CMRO2 to respond to CBF reduction, while it increases in cases with more decreased CMRO₂, as the AVDO₂ increase exceeds the CBF reduction to maintain the decreased CMRO₂ for a further CBF reduction. The vascular CO₂ reactivity, therefore, might be maintained to be constant between these patients.