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Objective: Ambulatory arterial stiffness index (AASI) has been recently proposed to reflect the dynamic relation between diastolic and systolic blood pressure throughout the whole day. The aim of our study was to investigate the change in AASI with advancing age and the correlation with 24 hours pulse pressure (24 h PP) in healthy individuals., Methods: 246 healthy subjects [mean age (59.7 +/- 14.6) years, women 38.6%] underwent 24 hours ambulatory blood pressure monitoring (ABPM) in normal life style. The blood pressure recordings, heart rate (HR), mean arterial pressure (MAP) and pulse pressure (PP) were analyzed simultaneously by computer for every 30 minutes during 6:00 am-22:00 pm and every 60 minutes during 22:00 pm-6:00 am. Using all the blood pressure recordings, we plotted diastolic against systolic blood pressure from each individuals and calculated the regression slope. AASI was derived from 1 minus this regression slope., Results: In 246 healthy individuals, AASI increased with age. Among the healthy individuals, the 95th percentile of AASI was 0.56, the upper boundary of the 95% prediction interval of AASI in relation to age were 0.49 at 20 - 39 years, 0.59 at 40 - 59 years, 0.69 at 60 - 79 years, 0.79 at > or = 80 years. The correlation coefficient between AASI and 24 h PP was 0.497 (P < 0.01). AASI linearly increased with age in healthy individuals, whereas the relation between pulse pressure and age was curvilinear., Conclusions: AASI as a index reflecting blood pressure relationship, manifested the corresponding change with advancing age. The correlation between AASI and traditional index 24 h PP indicated AASI as a new measure of arterial stiffness.