OBJECTIVES: We hypothesized that one minus the slope of diastolic on systolic blood pressure in individual 24-h ambulatory blood pressure recordings (ambulatory arterial stiffness index) might reflect arterial stiffness and predict cardiovascular mortality. METHODS: In volunteers and a population recruited in China, we studied concordance between ambulatory arterial stiffness index and established indexes of arterial stiffness. We assessed the predictive value of ambulatory arterial stiffness index in relation to cardiovascular outcome in Irish hypertensive patients. RESULTS: In 166 healthy volunteers, aged 22-83 years, the correlation coefficient between ambulatory arterial stiffness index and pulse wave velocity was 0.51 (P<0.001). In 348 randomly recruited Chinese, the correlations between ambulatory arterial stiffness index and both the central and peripheral systolic augmentation indexes were significantly stronger than those for 24-h ambulatory pulse pressure, particularly in study participants younger than 40 years. Among normotensive participants, the 95th percentile of the ambulatory arterial stiffness index was 0.55 in 234 Chinese and 0.57 in 1617 Europeans enrolled in the International Database on Ambulatory Blood Pressure Monitoring. The upper boundary of the 95% prediction interval of the ambulatory arterial stiffness index in relation to age ranged from 0.53 at 20 years to 0.72 at 80 years. In 11 291 patients enrolled in the Dublin Outcome Study, both ambulatory arterial stiffness index and 24-h ambulatory pulse pressure significantly predicted cardiovascular mortality. Ambulatory arterial stiffness index was a strong predictor of fatal stroke in normotensive participants, whereas pulse pressure better predicted heart attack in hypertensive patients. CONCLUSION: Ambulatory arterial stiffness index is a novel measure of arterial stiffness, which can be readily determined from ambulatory blood pressure recordings and which independently predicts cardiovascular mortality.