Background: Metabolic syndrome (MS) has been defined by the presence of 3 or more MS components among central obesity, elevated fasting glucose, triglycerides, blood pressure and decreased HDL (ATPIII, 2001), but this definition has been modified recently (2005). MS is of major public health concern, as most individuals with MS will die from cardiovascular diseases (CVD). The prevalence of MS is well characterized in many industrialized or developing countries; however, there is presently no estimate for Switzerland. Aims: We have previously shown that metabolic syndrome was associated with altered coronary circulatory vasomotion (a possible surrogate of cardiovascular event rate) in presence of some but not all components of the MS when using the ATPIII definition. Whether this was still the case with the new definition (2005) is unknown. Furthermore, to estimate the potential public health impact for Switzerland, we estimated MS prevalence from available data collected within a health promotion program run in the State of Vaud (Western Switzerland). Methods and results: Myocardial blood flow (MBF) was measured with PET and N-13-ammonia in 91 nondiabetic patients split in 4 groups with 0, 1, 2 and 03 components of the MS during cold pressor testing (CPT, mostly endothelium-dependent vasomotion) and dipyridamole (DIP, mostly endothelium-independent vasomotion). In a separate study in Switzerland, CVD risk factors were measured in 9,633 health program participants (aged 20-79y, 41%W) with a mobile unit (2001-4). MS prevalence was stratified by gender and age, allowing extrapolating the prevalence of MS in the Swiss population. The MBF response during CPT was significantly attenuated in presence of 2 or 03 MS components, while DIP MBF was preserved. Both CPT and DIP responses inversely correlated with the number of MS components. In Switzerland, the MS prevalence was 14.6% (>0.8 mio.; 17.8%M/9.8%W) with a strong age dependency (20-79y: 7-41%M; 2-34%W), significantly lower in women <60y. If we consider that patients with 2 components would already be at increased risk, this would affect 34% of the Swiss population (>1.8mio.; 41%M/24%W). Conclusion: Coronary vasomotion was diminished in MS and in presence of 2 MS components, with a graded decline in endothelium-dependent response suggesting cumulative negative effect of each MS component, eventually contributing to the increased CVD rate, which could affect a substantial part of the Swiss population.