A new simple criterion for diagnosing metabolic syndrome was proposed in the third report of the NCEP (National Cholesterol Education Program). In the present study, we analysed the association between metabolic syndrome and insulin resistance to investigate the effects of the latter on the prevalence of metabolic syndrome based on the new criteria recommended in the ATP (Adult Treatment Panel) III report. A total of 7057 participants (4472 men and 2585 women), who underwent medical screening at the Sungkyunkwan University Kangbuk Samsung Hospital, were investigated. Fasting insulin levels were measured and components of the metabolic syndrome as defined by the ATP III report were determined. We also applied the criteria for abdominal obesity as defined by APC-WC (Asia-Pacific criteria for waist circumference). The prevalence of metabolic syndrome as defined by ATP III was 5.3% (5.0% in men and 5.8% in women) and 8.9% (8.1% in men and 10.3% in women) by APC-WC. The odds ratio for the metabolic syndrome was significantly higher in subjects with higher insulin resistance than in those with lower insulin resistance. The mean levels of HOMA (homoeostatic model assessment) and fasting insulin were significantly higher in those with more of the components of the metabolic syndrome. A high HOMA (> or =2.56) and fasting insulin concentration (> or =9.98 microIU/ml; where IU is international unit) were found to be independent risk factors of the metabolic syndrome by multiple regression analysis after adjusting for age, sex and body mass index (P<0.001). These results show that the metabolic syndrome is significantly correlated with the insulin resistance index, and that appropriate values of HOMA and fasting insulin concentration may serve as a helpful guide for the management of metabolic syndrome.