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BACKGROUND: The ambulatory arterial stiffness index (AASI) is increasingly used in clinical research and practice. This individual-participant meta-analysis aims to consolidate the prognostic accuracy of AASI in the general population and to derive an end point-based AASI risk threshold. METHODS: In 12 558 individuals enrolled in 14 population studies (48.8% women; mean age, 59.3 years), AASI was derived by regressing 24-hour diastolic on systolic blood pressure (mm Hg/mm Hg). Using Cox regression, the risk-carrying AASI threshold was established by examining stepwise increasing AASI levels and by determining the AASI level, yielding a 10-year risk similar to an office systolic pressure of 140 mm Hg. RESULTS: Over 10.7 years (median), 3027 all-cause deaths and 2183 cardiovascular end points occurred. In all participants, multivariable-adjusted hazard ratios expressing the all-cause deaths and cardiovascular end point risk per 1-SD AASI increment were 1.08 (95% CI, 1.04-1.13) and 1.13 (95% CI, 1.07-1.18). In a randomly defined subset of 8189 individuals, the risk-carrying AASI thresholds converged to 0.50 with hazard ratios (>= 0.50 versus <0.50) of 1.14 (95% CI, 1.04-1.26) for all-cause deaths and 1.13 (95% CI, 1.01-1.26) for cardiovascular end point. In the replication sample (n=4369), these hazard ratios were 1.13 (95% CI, 1.01-1.26) and 1.19 (95% CI, 1.04-1.35). AASI continuous or per threshold significantly improved model performance. Analyses of secondary end points and subgroups stratified by sex, age, hypertension status and treatment, history of cardiovascular disease, and nocturnal dipping were confirmatory. CONCLUSIONS: Over and beyond traditional risk factors, AASI improves risk stratification. Exceeding the risk-carrying 0.50 AASI threshold necessitates increased vigilance in managing risk factors before irreversible cardiovascular complications occur.