The serum uric acid level has been said to be an independent predictor of cardiovascular disease death, mainly for women, and to be linked with the metabolic Syndrome X of insulin resistance, obesity, hypertension, and dyslipidemia. Recently, it has been suggested that the elevation of serum leptin, the ob gene product, may have a role in metabolic Syndrome X. Therefore, we studied the relationship of uric acid to leptin in 822 Japanese women in a cross-sectional manner. To estimate the effect of uric acid on the variables of metabolic Syndrome X, we calculated mean values of various components of the syndrome according to tertiles of uric acid (UA<4.0 mg/dl, 4.0≤UA<5.5, 5.5≤UA). Age, systolic and diastolic blood pressure (BP), body mass index (BMI), percent body fat mass (BFM), serum total cholesterol, triglyceride, atherogenic index, leptin, fasting immunoreactive insulin and homeostasis model assessment-ratio (HOMA-R: calculated insulin resistance) were significantly different across the uric acid tertiles with higher levels in the highest tertile in comparison to the first (ANOVA, p<0.001, 0.001, 0.002, 0.001, 0.001, 0.025, 0.001, 0.001, 0.001, 0.001, 0.001, respectively), while high density lipoprotein cholesterol showed lower levels (p<0.001). Serum leptin concentrations were also elevated in hyperuricemic women after adjusting for BMI or BFM (both p<0.001), and were weakly correlated with serum uric acid concentrations (r=0.22, p<0.0001). BMI, HOMA-R, serum triglyceride, diastolic BP and age-adjusted serum leptin concentrations were calculated for each tertile of serum uric acid. Compared with the lowest tertile of uric acid level, BMI, HOMA-R, serum triglyceride, diastolic BP and age-adjusted leptin concentrations were higher in the highest tertile. In the stepwise regression analysis, serum leptin was the significant independent variable for uric acid values. These results indicate an independent relationship between leptin and uric acid, further supporting the involvement of leptin in metabolic Syndrome X.