The aim of this study was to compare the prevalence of the metabolic syndrome diagnosed using three different definitions and to evaluate its associations with ischemic heart disease in Kaunas adult population. MATERIAL AND METHODS. Data of preventive screening carried out in Kaunas in 2001-2002 according to the MONICA study protocol were used for analysis; a total of 1336 persons aged 35-64 years (603 men and 733 women) were recruited. The metabolic syndrome was defined by the World Health Organization, Adult Treatment Panel III, and International Diabetes Federation definitions. Ischemic heart disease was diagnosed based on the following criteria: a documented history of myocardial infarction, angina pectoris, or ischemic changes on electrocardiogram. RESULTS. The metabolic syndrome was identified for 11.3% of men and for 9.4% of women using the World Health Organization definition, for 19.4% of men and for 26.3% of women using the Adult Treatment Panel III definition, and for 30.0% of men and for 37.7% of women using the International Diabetes Federation definition. In male and female groups, the prevalence of the metabolic syndrome (irrespective of definition) significantly increased with age (P<0.05). After adjusting for age, men diagnosed with the metabolic syndrome using the International Diabetes Federation definition (OR=2.30; P=0.001) and Adult Treatment Panel III definition (OR=1.97; P=0.01) and women diagnosed with metabolic syndrome using the International Diabetes Federation definition (OR=1.50; P=0.039) had a significantly higher risk of having ischemic heart disease as compared with those without the metabolic syndrome by the same definitions. The metabolic syndrome diagnosed using the World Health Organization definition was not associated with a significant risk of ischemic heart disease in men and women. CONCLUSION. In Kaunas population aged 35-64 years, the highest prevalence of the metabolic syndrome was determined according to the International Diabetes Federation definition. Usage of the International Diabetes Federation and the Adult Treatment Panel III definitions in establishing diagnosis of the metabolic syndrome provides more opportunities to identify subjects with ischemic heart disease.