Leptin, the product of ob gene is secreted by adipose tissue. It is believed that leptin plays an important role in energy balance. The secretion of leptin by adipose tissue is influenced by insulin. The aim of the present study was the estimation of plasma leptin concentrations in patients with type 2 diabetes mellitus. The study was carried out in 21 diabetic obese patients (BMI > 27.5), 8 diabetic patients with BMI < 27.5, 24 obese patients with normal glucose tolerance (BMI > 27.5) and 10 patients from the control group (BMI < 27.5). The mean leptin concentration in obese diabetic patients was 22.5 + 6.5 ng/ml and was not significantly different from that in obese patients without diabetes (24.1 + 10.3 ng/ml) but differed markedly in comparison to the normal weight diabetic patients (7.9 + 4.3 ng/ml, p < 0.01). Plasma leptin concentration correlated significantly and positively with BMI and fasting insulin in all studied groups. There was no significant correlation between leptin and glycated hemoglobin, total cholesterol and triglycerides. We conclude that serum leptin concentrations in patients with type 2 diabetes depends mainly on the amount of body fat.