BACKGROUND: To determine whether hyperleptinemia is a principal component of metabolic syndrome in a Korean population using factor analysis. METHODS: Metabolic syndrome was defined by the NCEP-ATP III guideline. An oral glucose tolerance test was performed, and plasma samples for leptin and lipid profiles were collected from 199 men and 426 women who had no history of diabetes, hypertension, dyslipidemia, or of taking lipid-lowering, antihypertensive, or antihyperglycemic medications. RESULTS: Leptin level was correlated with overall and central obesity, blood pressure, and glucose or insulin levels in men and women aged 30 to 83. Before and after adjustment for BMI, leptin level was significantly and positively correlated, in women only, with insulin and with insulin resistance, as assessed by a homeostasis model assessment (HOMA) (Ps<0.0001). Factor analysis identified the following four factors from among the metabolic syndrome variables; an obesity/hyperinsulinemia factor, a glucose intolerance factor, a hypertension factor, and a dyslipidemia factor in men. Leptin was clustered as an obesity/ hyperinsulinemia and a dyslipidemia factor in men. In women, four different groups were found: an obesity/hypertension factor, a glucose intolerance factor, an obesity/dyslipidemia factor, and an obesity/hyperinsulinemia factor. Leptin was clustered as an obesity/hyperinsulinemia factor in women. CONCLUSION: Our research suggests that leptin level is associated with metabolic syndrome in relation to obesity and hyperinsulinemia. Moreover, obesity, as opposed to hyperinsulinemia, is related to hypertension or dyslipidemia in women only, and this gender differences may reflect different roles of central adiposity on metabolic abnormalities.