The effects of long-term supervised aerobic exercise on the body weight, serum lipids and high density lipoprotein cholesterol (HDL-C) were studied in 41 obese children (21 boys, 20 girls) initially aged 11-years-old. The 2 year training program was performed during the daily school life. The intensity of training consisted of running 20 minutes seven times per week at a pace which corresponds to the blood lactate threshold. No dietary intervention was done. The obesity index significantly decreased after 1 year of aerobic exercise in both boys and girls by 55 percent and 48 percent respectively. Since the lean body mass increased throughout the 2 year study while the total body weight decreased, the weight loss is attributed to a decrease in fat. The concentration of HDL-C significantly increased in both boys and girls in the first year, 16 percent and 19 percent respectively and was a slightly lower value in the second year. After 2 years a significant reduction in the girls' serum triglyceride concentration was observed. However, the total cholesterol in serum was unaltered in either boys or girls throughout the 2 year study. In conclusion, a long-term supervised aerobic exercise program in obese children is beneficial and resulted in significant weight reduction with concomitant improvement of lipoprotein metabolism.