To determine the pathophysiological implications of serum leptin level in obesity, we monitored the changes in serum leptin level during outpatient treatment with life style modification in children. Fifty-five obese Japanese children (34 boys and 21 girls; mean age, 9.64 years) were studied. The control children consisted of 42 nonobese subjects (27 boys and 15 girls). The serum leptin concentration was 4.35±0.46ng/ml (mean±SEM) in the control girls and 2.93±0.21ng/ml in the control boys. The serum leptin concentrations in the obese boys and girls were higher than those in their lean counterparts. The concentration in the obese boys (16.28±1.41ng/ml) was similar to that in the obese girls (20.33±2.0ng/ml). The logarithmic value of serum leptin concentration at the first blood sampling in obese children was correlated with percent overweight and percent body fat. In 36 obese children (24 boys and 12 girls) whose serum leptin concentrations were monitored serially during treatment of obesity, the percent overweight was significantly decreased after the initial sampling. In each individual, the changes in leptin concentration were roughly parallel to those in percent overweight. The ratio of the leptin concentration at the second blood sampling divided by the one at the first sampling in each individual was closely correlated with the respective delta percent overweight. These results suggest that the preceding course of obesity determines the serum leptin level of obese children on longitudinal basis, and that the leptin level reflects the degree of obesity on cross-sectional basis.