The authors evaluated the effects of superficial temporal to middle cerebral artery (STA-MCA) bypass on CO2 cerebrovascular reactivity (CVR) in ischaemic cerebrovascular diseases (CVDs). Local cerebral blood flow (LCBF) and CO2 CVR in 19 patients with ischaemic CVD subjected to standard STA-MCA bypasses were examined during surgery. Single photon emission computed tomography (SPECT) with acetazolamide (ACZ) activation was also performed before and at 1 month after surgery. The results are as follows. 1) Before bypass, the average CO2 CVR value was -1.50 +/- 2.30%/mmHg (mean +/- SD). SPECT showed disturbed response to ACZ in all cases. Fifteen cases showed the steal phenomenon. After bypass, the mean CO2 CVR value significantly (p < 0.05) increased, and four cases resolved their steal phenomenon. 2) Before bypass, the mean LCBF was significantly (p < 0.05) lower than the control level. After bypass, the mean LCBF significantly (p < 0.05) increased. 3) In the postoperative SPECT findings, 13 cases showed a disturbed response to ACZ. The CO2 CVR value in these 13 cases was -1.21 +/- 1.19%/mmHg, which was significantly (p < 0.05) low compared to the values for the 6 cases showing normal postoperative ACZ responses. In ischaemic CVDs before bypass, the CO2 CVR values were extremely low. After bypass, however, CO2 CVR and LCBF values significantly improved. SPECT findings, including ACZ challenge, correlated well to the LCBF and CO2 CVR values. STA-MCA by pass exerted a favourable effect on the CO2 CVR and LCBF values immediately after bypass in the cases showing a reduced pre-operative response to CO2.