PURPOSE: Leptin is a product of ob gene in adipose tissue. This is a important factor for regulation of body weight, especially body fat mass. So it is interested in correlation of leptin with obesity or various diseases related with obesity, particularly diabetes mellitus. It was demonstrated that leptin level was increased in type 2 diabetes and insulin was associated with increased leptin level. This study is to measure leptin levels in type 1 diabetes without insulin resistance, to validate whether insulin increases leptin level and to investigate correlation of leptin with degree of diabetic control and pubertal progression. METHODS: This study included 114 children(boys; 57, girls; 57) with type 1 diabetes, whose blood samples were taken to measure leptin level using RIA(radioimmunoassay). We observed the correlation of leptin with body mass index, HbA1c, insulin dose, duration of disease, and pubertal degree. RESULTS: Mean values of age, BMI, insulin dose, and HbA1c at the measurement of leptin were 13.0+/-5.04 years-old, 18.9+/-3.5kg/m2, 36U/day, and 8.9+/-2.1%, respectively. Thirty seven children were prepubertal and fifty seven children had pubertal characteristics and the rests were not confirmed. With increment of BMI, serum leptin levels were significantly increased in the all subjects, with the same correlation in pubertal female group, but not in pubertal male. There was a relatively weak correlation(r=0.29) of leptin levels with insulin doses. In prepubertal male, there was a significant positive correlation(r=0.55) of leptin with insulin dose, whereas in pubertal male there was shown negative correlation(r=-0.41). Female patients had a significantly higher leptin levels than male, irrespective of pubertal changes or BMI. There was not a significant correlation of leptin levels with duration of diabetes and HbA1c level. CONCLUSION: It is suggested that blood levels of insulin and sex hormones play an important roles in determining the levels of leptin.