Serum levels of total cholesterol, triglycerides, lipoproteins, lipid peroxides (TBARS) and erythrocyte antioxidant enzyme activities were measured in 105 non insulin dependent diabetic patients, among whom 38 had microvascular complications (MVC) of diabetes. All the diabetic patients had higher concentrations of glycated hemoglobin (HbA1) compared to controls (10.51 +/- 2.42% vs 6.31 +/- 0.85% P <0.001). Significant increase of serum triglycerides (TG), total cholesterol (TC), low density lipoprotein cholesterol (LDL-C) and very low density lipoprotein cholesterol (VLDL-C) and a significant decrease of high density lipoprotein cholesterol (HDL-C) were observed in the diabetic patients compared to controls (TG: 2.31 +/- 0.9 mmol/l vs 1.53 +/- 0.48 mmol/l P <0. 001; TC: 5.94 +/- 1.4 mmol/l vs 4.3 +/- 0.85 mmol/l P <0.001; LDL-C: 3.96 +/- 1.33 mmol/l vs 2.39 +/- 0.8 mmol/l P <0.001; VLDL-C: 0.46 +/- 0.2 mmol/l vs 0.3 +/- 0.09 mmol/l P <0.001; HDL-C: 0.81 +/- 0.24 mmol/l vs 1.04 +/- 0.18 mmol/l P <0.001). Significantly increased levels of serum TBARS were observed in diabetic patients compared to those in controls (TBARS: 6.7 +/- 1.5 mmol/l vs 5.14 +/- 0.61 mmol/l P <0.001). Erythrocyte catalase (CAT) activity was increased and Glutathione peroxidase (GPx) activity was decreased in diabetic patients compared to controls, but no significant change in Superoxide dismutase (SOD) activity was observed in diabetic patients (CAT: 104.94 +/- 37.1 KU/g Hb vs 85.8 +/- 23.6 KU/g Hb, P <0.01; GPx: 30 +/- 9.7 U/g Hb/min vs 40.84 +/- 12.3 U/g Hb/min, P <0. 001; SOD: 2.4 +/- 1.2 U/mg Hb/min vs 2.55 +/- 0.84 U/mg Hb/min, P=NS). In comparison with the diabetic group without MVC, the diabetic group with MVC had decreased GPx and SOD activities, while no difference was observed between these two groups regarding CAT activity (GPx: 25.32 +/- 8.4 U/g Hb/min vs 34.5 +/- 8.8 U/g Hb/min, P <0.001; SOD: 1.83 +/- 0.53 U/mg Hb/min vs 2.84 +/- 1.4 U/mg Hb/min, P<0.001; CAT: 106.3 +/- 39.9 KU/g Hb vs 103 +/- 34.9 KU/g Hb, P =NS). TBARS concentrations were significantly increased in the group with MVC compared to the group without these complications, indicating a positive relationship between TBARS and MVC of diabetes (7.05 +/- 1.23 mmol/l vs 6.3 +/- 1.02 mmol/l, P <0.001). Serum triglycerides, LDL and VLDL cholesterol concentration were significantly higher in diabetics with MVC than in diabetics without the complications (TG: 2.7 +/- 0.98 mmol/l vs 2.13 +/- 0.82 mmol/l, P<0.01; LDL - C: 4.45 +/- 1.3 mmol/l vs 3.67 +/- 1.3 mmol/l, P <0. 02; VLDL-C: 0.53 +/- 0.19 mmol/l vs 0.43 +/- 0.16 mmol/l, P <0.01), and the serum levels of TC in the group with MVC showed a positive correlation with their lipid peroxide levels (r =0.368, P <0.001). The increase in TBARS and the decreased GPx and SOD activities in diabetics with MVC in this study indicate that these factors may contribute to the occurrence of micro vascular complications in NIDDM patients.