The effects of belonging to sports clubs on male high school students was evaluated. The relationships between the type and extent of school-based exercise were examined in conjunction with percent body fat, blood pressure (BP), and other key metabolic parameters. A total of 264 male Japanese high school students (age range: 17-18 years old) were studied. Percent body fat was measured and blood was collected in the fasting state during a routine health check. Subjects were divided into two groups. The exercise (E) group (n=150) included students who had belonged to a sports club during the past 2 years. The non-exercise (NE) group (n=114) included students who did not belong to a sports club during the past 2 years. The body mass index was significantly greater in group E (21.7±2.3 (SD) kg/m²) than in group NE (20.7+ 2.6kg/m², p<0.01). However, the percent body fat in group E (13.6±3.4%) was significantly lower than that in group NE (14.9±3.8%, p<0.01). The diastolic BP and heart rate in group E (64±7mmHg, 70± 11/min) were significantly lower in group E than in group NE (66±8mmHg, p<0.05; 76±14/min, p< 0.01). The serum triglyceride level was significantly lower, and the HDL cholesterol level was higher in group E than in group NE. The homeostasis model assessment (HOMA) index, used as an index of insulin resistance, was similar in the two groups. However, the level of the HOMA index was significantly lower among the 62 subjects in group E who preferred highly dynamic exercise (1.50±0.46) than it was among those in group NE (1.66±0.49, p<0.05). Results indicate that belonging to sports clubs influences the BP and lipid profiles of adolescent males, as well as their percent body fat. In view of the reduction of cardiovascular risk factors, it is recommended that even young males practice regular exercise, especially aerobic exercise. (Hypertens Res 2000; 23: 227-232)