Increased arterial stiffness has been shown to predict cardiovascular risk in hypertensive patients. Our objective was to evaluate the relationship between the ambulatory arterial stiffness index (AASI) and subclinical organ damage (SOD). The design was a cross-sectional study. Subjects included 554 hypertensive patients with and without drug treatment (mean age 57±12 years, 60.6% men). The AASI was defined as 1 minus the regression slope of diastolic over systolic blood pressure (BP) readings obtained from 24-h recordings. Renal damage was evaluated on the basis of glomerular filtration rate (GFR) and microalbuminuria; vascular damage was measured by carotid intima-media thickness (IMT) and ankle/brachial index (ABI); and cardiac damage was evaluated on the basis of the Cornell voltage-duration product (VDP) and left ventricular mass index. The mean AASI was 0.38±0.07 (0.39±0.07 in treated patients and 0.37±0.06 in nontreated subjects). The AASI showed a positive correlation with IMT (r=0.417, P<0.001) and Cornell VDP (r=0.188, P<0.001), and a negative correlation with GFR (r=-0.205, P=0.001) and the ABI. The variables associated with the presence of SOD were AASI (odds ratio (OR)=3.89) and smoking (OR=1.55). The variables associated with IMT were smoking and waist circumference, whereas those associated with GFR were AASI, body mass index and waist circumference. In turn, smoking, total cholesterol and glycosylated hemoglobin A1c were associated with the ABI. Increased AASI implies a greater presence of SOD in primary hypertensive patients with or without BP-lowering drug treatment.