The association between different features of the metabolic syndrome (MS) (obesity, hypertension, hypercholesterolemia, low high-density lipoprotein cholesterol level, hypertriglyceridemia, high fasting plasma glucose level, and hyperuricemia) and the risk for increased aortic pulse wave velocity (PWV) of > or = 8.0 m/sec was examined in 2431 Japanese men aged 35 to 54 years who were not taking antihypertensive medication. After controlling for age, cigarette smoking, and alcohol intake, the odds ratios for increased aortic PWV in subjects with 1, 2, 3, and > or = 4 features of the MS, compared with those without features of the MS, were 1.35 (95% CI, 0.86 to 2.11), 1.90 (95% CI, 1.18 to 3.06), 1.57 (95% CI, 0.89 to 2.76), and 2.38 (95% CI, 1.26 to 4.49), respectively (p for trend = 0.003). A 9-year longitudinal study was also performed to prospectively examine the association between clustered features of the MS and the development of increased aortic PWV in 2073 men without aortic stiffness with a PWV < 8.0 m/sec and without antihypertensive medication during the follow-up period. The multivariate-adjusted hazard ratios for the incidence of increased aortic PWV in subjects with 1, 2, 3, and > or = 4 features of the MS, compared with those without features of the MS, were 1.39 (95% CI, 1.10 to 1.77), 1.46 (95% CI, 1.1 1 to 1.92), 1.75 (95% CI, 1.27 to 2.40), and 2.22 (95% CI, 1.52 to 3.25), respectively (p for trend < 0.001). These results suggest that clustered features of the MS are closely associated with the risk for increased aortic PWV in middle-aged Japanese men.