Leptin values were investigated during four week treatment of obese children by weight reducing diet and dosed physical activity, controlled by sport-testers. Leptin values correlated significantly with BMI and HDL-cholesterol at the beginning of spa treatment and at the end of treatment. Values of leptin at the beginning of treatment correlated with fasting insulinaemia. There were no changes in leptin values during oral glucose tolerance test. Significant decrease of BMI, total cholesterol, LDL-cholesterol, triglycerides, systolic BP and leptin values appeared after four week treatment. Leptin values did not correlate with total or LDL-cholesterol at the beginning or at the end of the treatment and leptin values were not predictive for development of atherosclerosis.