Background Not only the obese,but also the non-obese adults have the high prevalence of metabolic syndrome in the upper normal weight.The aim of this study was to assess the prevalence rates of metabolic syndrome and its individual components in non-obese adult Taiwanese(body mass index(BMI)≤26.9 kg/m~2).Methods A cross-sectional study was conducted from January 2006 to December 2007.One thousand six hundred and fifty-nine subjects(aged 47.5±12.4 years),60.8% of which were men,were enrolled.The prevalence and odds ratios of metabolic syndrome,defined by the American Heart Association/National Heart,Lung and Blood Institute(2005),were analyzed in the BMI category according to 2.0 unit increments,in individuals seeking a health examination.Results The higher the BMI categories,the more prevalent the metabolic syndrome was in women and in men(P＜0.001).Compared with those women with a BMI≤20.9 kg/m~2,the odds ratios for metabolic syndrome in women were 1.3(95% CI:0.5-3.2)with BMI 21.0-22.9 kg/m~2,3.0(1.3-7.1)with BMI 23.0-24.9 kg/m~2,and 8.6(3.6-20.8)for women with BMI 25.0-26.9 kg/m~2,after controlling for age,smoking status,alcohol consumption,betel nut chewing,blood routine,biochemical data,hepatitis B virus surface antigen and anti-hepatitis C virus.The corresponding odds ratios in men were 1.6(0.6-4.2),3.7(1.6-8.8),and 9.9(4.2-23.2).Conclusions Individuals in the upper normal weight and slightly overweight BMI range have relatively high prevalence and increased risk of having metabolic syndrome.Therefore,physicians should screen metabolic syndrome in not only obese but also non-obese individuals for the prevention of cardiovascular disease.