OBJECTIVE To evaluate the relationship between serum levels of leptin and insulin in prepubertal lean, obese and insulin-dependent diabetes mellitus (IDDM) children. METHODS Prepubertal children, 16 lean, 17 obese and 16 IDDM were included in the study. Fastang serum leptin and insulin concentrations were measured by radioimmunoassays. RESULTS The serum level of leptin was significantly higher in obese children than in lean and IDDM children (P < 0.0001 and P < 0.0001, respectively), and showed a positive correlation with body mass index (BMI) for the combined group (lean, obese and IDDM; r = 0.77, P < 0.0001). In addition, the serum leptin level was higher in IDDM children than in lean controls (P < 0.01), whereas no difference was found in BMI between the two groups. The mean fasting serum levels of insulin were significantly elevated in IDDM children as compared with lean controls (P < 0.01). A significant positive correlation was found between serum insulin and leptin levels for the combined group (r = 0.37, P < 0.01). When a multiple regression analysis for all subjects was performed, the total contribution of all parameters, including gender, BMI and log insulin, accounted for 75% of the leptin variation. BMI (57.8%), log insulin (14.0%) and gender (3.2%) contributed significantly to this variation. CONCLUSIONS The elevated concentration of leptin in insulin-dependent diabetic children, independent of body mass index, was probably caused by chronically increased serum insulin levels. We demonstrated that not only body mass index but also insulin was a significant independent predictor of serum leptin concentrations. It is therefore suggested that insulin might play an important role in regulating serum leptin concentrations independent of adiposity.