This study illustrates the relationship between oxidative DNA damage and obesity in patients with prediabetes and type 2 diabetes compared with controls.Participants attended the School of Community Health, Diabetes Screening Clinic, Charles Sturt University, Australia, between February 2006 and June 2008. A total of 162 participants (35 type 2 diabetic patients; eight prediabetic subjects; and 119 age-, gender-, and weight-matched controls) were investigated. All patients were selected on clinical grounds.Serum 8-hydroxy 2'-deoxy-guanosine (8-OHdG) level was significantly greater in the prediabetic subjects (671.3±140 pg/ml) compared with controls (210.1±166 pg/ml; P<0.01). The diabetic group (1979.6±1209 pg/ml) had the highest level of 8-OHdG. There was a significant increase in serum 8-OHdG in obese subjects (848.5±103 pg/ml; P<0.001) and overweight subjects (724±102 pg/ml; P=0.005) compared with the lean subjects (196.5±327 pg/ml).Our results indicate that serum 8-OHdG is increased already in prediabetes suggesting oxidative DNA damage to be present with minor elevation of blood glucose levels (BGLs). The statistically significant positive correlation between serum 8-OHdG and body mass index in the diabetic group indicates that obesity has an additive effect to increased BGL contributing to oxidative DNA damage.