The aim of this study is to clarify the influence of aging on serum leptin concentration.We measured serum concentration of leptin in 279 outpatients (130 males and 149 females), aged from 36 to 96 years. The subjects were divided into three groups according to their age; Group A (<60 yr.), Group B (60-69 yr.), and Group C (≥ 70 yr.). The mean BMI values were not significantly different among three groups both in men and in women, and these are not significantly different between men and women in all three groups. The mean serum leptin concentrations were not significantly different among three groups in men, but these were significantly lower in Group C than either Group A and Group B in women. The mean serum leptin concentration were significantly higher in women than in men both Group A and Group B, but these were not significantly different in Group C. Significant positive correlation was found between serum leptin concentrations and BMI both in men and in women. Significant positive correlation was found between serum leptin concentrations and serum triglyceride concentrations in women group, but was not found in men. Significant positive correlation was found between serum leptin concentrations and serum VLDL-C concentrations in Group A (r = 0.64) or Group B (r = 0.51) in women, but was not found in men. In the men, serum leptin concentrations positively correlated with serum IRI concentrations in Group A (r = 0.45) or Group B (r = 0.78). In the women, serum leptin concentrations positively correlated with serum IRI concentrations in Group C (r = 0.47). In conclusion, difference in serum leptin levels by sex trends to weaken with aging.