Leptin, a product of the ob gene, is a polypeptide hormone produced in adipose tissue that informs the brain about the amount of energy storage of body fat. It has very important effects on neuroendocrine functions and energy expenditure. The aim of our study was to determine leptin levels of children with insulin dependent diabetes mellitus (IDDM), which is known to affect body metabolism, and to investigate the relationship between duration of the disease, insulin dosage, HbA1c levels, body mass index (BMI), serum lipids and IGF-1 levels. Sixteen patients with IDDM (chronological age 13.8 +/- 2.6 years) whose HbAlc levels were 10.2 +/- 1.9 %, BMI 21.2. +/- 2.7 kg/m2, insulin dosage 0.9 +/- 0.4 U/kg/day and duration of the disease 6.7 +/- 2.6 years, and 12 healthy controls (13.4 +/- 2.6 years) were included in the study. Fasting plasma leptin levels were measured by radioimmunoassay method. The mean plasma leptin levels of the patient and the control groups were 19.1 +/- 7.6 ng/ml and 6.1 +/- 2.9 ng/ml, respectively, and significant difference was found between the two groups (p < 0.05). No correlation was found between leptin values and IGF-1, cholesterol, HDL-cholesterol, LDL-cholesterol, triglyceride levels, atherogenic index, insulin dosage or HbA1c levels in the patient group. A weak statistical correlation was determined between BMI and leptin levels in the IDDM group (r = 0.28, p < 0.05). A positive correlation was also found between leptin levels and the duration of the disease (r = 49, p < 0.05). As a result, it seems that leptin levels of children with IDDM differed from the levels of the control group significantly, and that the duration of insulin therapy was responsible for this difference.