In this study, we aimed to examine the impact of the metabolic syndrome and its components on the risk of cardiovascular disease among a relatively less-obese population. A total of 8249 men and 15 064 women, aged 40-69 years, with no history of ischemic heart disease, stroke and/or cancer completed a risk-factor survey between 1993 and 1995. The metabolic syndrome was defined based on modified criteria of the American Heart Association/National Heart, Lung, and Blood Institute (AHA/NHLBI) and the International Diabetes Federation (IDF). Systematic cardiovascular surveillance was carried out throughout 2003, and 693 events of ischemic heart disease and stroke were identified. We observed significant associations of the metabolic syndrome with the risk of ischemic heart disease and ischemic stroke, but not with hemorrhagic stroke. The multivariable hazard ratio (95% confidence interval) of ischemic heart disease among men for the metabolic syndrome based on the AHA/NHLBI criteria was 2.25 (1.44-3.51) and that of ischemic stroke was 1.88 (1.40-2.52). The respective hazard ratios for the metabolic syndrome based on the IDF criteria were 1.61 (0.99-2.64) for ischemic heart disease and 1.94 (1.41-2.68) for ischemic stroke. The population-attributable fraction (PAF) of the metabolic syndrome based on the AHA/NHLBI criteria was higher than that based on the IDF criteria: 19 vs. 12% (P for difference=0.003) for ischemic cardiovascular disease among men, because non-overweight men with >or=2 risk factors were also at high risk (20% of the PAF). Our data suggest that the metabolic syndrome based on the AHA/NHLBI criteria predicts ischemic cardiovascular disease better than the syndrome based on the IDF criteria, because of the exclusion of non-overweight high-risk individuals from the reference group.