The 2 most widely used criteria to diagnose the metabolic syndrome (MS) are those developed by the United States Adult Treatment Panel III of the National Cholesterol Education Program (ATP III) and by the International Diabetes Federation (IDF). A major difference between these 2 sets of criteria is that the IDF places more emphasis on waist circumference. We compared the prevalence of MS using the ATP III and the IDF guidelines in 2 American (the Dallas Health Study and National Health and Nutrition Examination Survey) and 1 German (Prospective Cardiovascular Munster study) population samples. When the ATP III criteria were used, the prevalence of MS was higher in the United States than the German samples in both women (37% vs. 18%) and men (30% vs 25%), whereas when the IDF criteria were used, the prevalence of MS was 25% higher in the German than the American sample. Although in the United States both sets of criteria identified mostly the same people (concordance of about 90%), this was less true in Germany (concordance about 80%). To determine which criteria better predicted adverse cardiovascular outcomes, the incidence of coronary events associated with MS, as defined using the ATP III or the IDF criteria, were compared over a 10-year period among the middle-aged men in the German sample (n = 7,152). A total of 3.4% of men without MS developed an event. A much higher percentage of the men with MS defined by the ATP III criteria (10.7%) than the IDF criteria (5.5%) had a cardiovascular event. In conclusion, although the prevalence of MS was higher when the IDF criteria were used in the German sample, the IDF criteria have lower predictive power for coronary events.