We hypothesized that the dynamic relation between diastolic and systolic blood pressure over 24 hours provides a measure of arterial stiffness and might, therefore, predict cardiovascular mortality over and above pulse pressure. At baseline, while not on antihypertensive medication, 11 291 patients (mean age, 54.6 years; 5965 women) underwent ambulatory blood pressure monitoring. Using all of the blood pressure readings, we plotted diastolic against systolic blood pressure from each individual and calculated the regression slope. The ambulatory arterial stiffness index (AASI) was defined as 1 minus this regression slope. Over a median follow-up of 5.3 years, 566 cardiovascular deaths occurred, including 151 from stroke and 358 from cardiac disorders. Before and after adjustment for other cardiovascular risk factors, AASI and pulse pressure significantly predicted total cardiovascular mortality. AASI was a stronger predictor than pulse pressure for stroke (mutually adjusted relative hazard ratios for 1 SD increase, 1.21 versus 1.04; P=0.02 versus 0.66) with the opposite trend for cardiac mortality (relative hazard ratios, 1.03 versus 1.21; P=0.63 versus 0.002). In subjects with normal daytime ambulatory blood pressure (<135/<85 mm Hg), AASI was more predictive than pulse pressure of cardiovascular mortality (1.26 versus 0.96; P=0.04 versus 0.70) and of stroke mortality (1.81 versus 1.12; P=0.007 versus 0.58), whereas neither independently predicted cardiac mortality (1.11 versus 0.89; P=0.47 versus 0.40). AASI is a novel measure of arterial stiffness, which can be readily determined from ambulatory blood pressure recordings and which independently predicts cardiovascular mortality, even in normotensive subjects.