rCBF was measured by 133Xenon inhalation at rest and during pCO2 manipulation in 19 patients prior to and at various times after STA-MCA bypass surgery. The resting flow increased gradually postoperatively, seemingly more due to progressive clinical recovery from the original neurologic disability than due to the surgical intervention. CO2 reactivity increased in some patients, more in those who preoperatively had the poorest reactivity. The negative relationship between preoperative reactivity and postoperative change in reactivity was highly significant. These observations are consistent with the interpretation that the surgery did not significantly affect the resting flow level but did augment the collateral reserve in those cases in which it had been most severely impaired by the arterial lesion.