The purpose of this study was to clarify the relationships between obesity (BMI) and BP levels, leptin levels, sympathetic activity, and insulin sensitivity in a Japanese male population. In 912 young, non-diabetic, Japanese men with a wide range of BMI (16.5-33.6kg/m²), blood pressure (BP), fasting plasma norepinephrine (NE), insulin and leptin levels were measured after an overnight fast. The cohort consisted of 603 normotensive and 309 hypertensive subjects. The study was carried out using a cross-sectional design. When the subjects were subdivided by tertile in relation to BMI, the 101 subjects in the heaviest group (BMI>27.9kg/m²) had a significantly higher systolic BP (p<0.05) and pulse rate (p<0.05) as well as higher NE (p<0.01), insulin (p<0.01), and leptin (p<0.01) levels than 86 subjects in the leanest group (BMI<22.2kg/m²). In the whole cohort, BMI correlated with mean BP (p<0.01), plasma NE (p<0.05), insulin (p<0.001) and leptin (p<0.001). The mean BP correlated with BMI (p<0.001), plasma NE (p<0.01), insulin (p<0.01) and leptin (p<0.05). Plasma leptin levels correlated with fasting plasma insulin levels (p <0.05), but not with plasma NE levels (NS). As analyzed by multiple regression analysis, only plasma NE (p<0.05) and BMI (p<0.001), but not plasma insulin levels, were significant, independent predictors of BP levels (r²=0.125, F=10.51, p=0.0001). These results suggest that obesity (BMI) and heightened sympathetic nervous system activity contribute to BP elevation (hypertension). (Hypertens Res 2000; 23: 303-310)