The aim of this study was to determinate both the oxidative stress/anti-oxidative defense status and the concentration of leptin in obese, overweight and normal weight type 2 diabetes mellitus patients to seek possible association between oxidative stress and hyperleptinemia. Oxidative stress status parameters [thiobarbituric acid-reacting substances (TBARS), superoxide anion (O(2)(-)), superoxide dismutase (SOD) activity and total sulphydryl groups] and the concentration of leptin were measured in 312 subjects (178 patients and in 134 control subjects). Obese patients had a significantly higher concentration of leptin compared to obese subjects in the control population (P<0.001). They also had significantly higher plasma concentrations of TBARS, O(2)(-) and SOD activity in combination with a lower sulphydryl group concentration when compared to control subjects. Obese patients had significantly higher concentrations of both TBARS and O(2)(-) and increased SOD activity compared to normal weight patients. The odds ratio for the degree of association between oxidative stress status parameters and hyperleptinemia was strongest for TBARS [odds ratio 2.66, 95% CI (1.02-6.94), P=0.045]. The observed positive correlation between TBARS and leptin (rho=0.29, P<0.01) in obese patients suggests that increased oxidative stress and hyperleptinemia, both consequences of obesity, may play a role in type 2 diabetes mellitus development.