Background--Pulse pressure, the ambulatory arterial stiffness index (AASI), and the symmetric AASI are established predictors of adverse cardiovascular outcomes. However, little is known about their relationship to cerebral autoregulation. This study evaluated whether these markers of vascular properties relate to the lower limit of cerebral autoregulation (LLA). Methods and Results--The LLA was determined during cardiac surgery with transcranial Doppler ultrasonography in 181 patients. All other variables were calculated from continuous intraoperative readings obtained before cardiopulmonary bypass. The LLA varied directly with the AASI (β=3.12 per 0.1 change in AASI, P < 0.001) and to a lesser extent the symmetric AASI (β=2.02 per 0.1 change in symmetric AASI, P≤0.022), while peripheral pulse pressure was not significantly related (β=0.0, P > 0.99). Logistic regression revealed that the likelihood of LLA being > 65 mm Hg increased by 50% (95% confidence interval, 11%-102%, P=0.008) for every 0.1 increase in the AASI. The AASI was able to predict a LLA above certain thresholds (area under the curve receiver operating characteristic for AASI predicting an LLA > 65 mm Hg: 0.60; 95% confidence interval, 0.51%-0.68%, P = 0.043). Incorporating additional variables improved the model's predictive ability (area under the curve for AASI predicting a LLA > 65 mm Hg: 0.75; 95% confidence interval, 0.68-0.82, P=0.036). Conclusions--These data indicate that the LLA is related to the mechanical properties of the vasculature as represented by the AASI. The AASI can be used to predict LLA threshold levels during cardiac surgery. It is now possible to link elevations in the LLA with an increased AASI as determined from readily accessible intraoperative variables.