An increased lipid peroxidation, due to the altered intracellular ratio between free radicals and antioxidant systems, has been recently related to diabetes. To study the possible relationship between lipid peroxidation and metabolic control, we measured the plasma concentrations of malondialdehyde (MDA), end product of the oxidation of polyunsaturated fatty acids, in poorly and well controlled Type 2 diabetic patients. A significant increase in plasma malondialdehyde concentrations was found in poorly controlled diabetics when compared to well controlled patients (p < 0.001) and to healthy normoglycaemic subjects (p < 0.001), whereas no significant difference was observed between the two latter groups. Plasma MDA/Cholesterol and MDA/triglyceride ratios were both higher in poorly controlled diabetics than in well controlled (p < 0.005) and in normal subjects (p < 0.01 and p < 0.02 for MDA/CHOL and MDA/TG respectively). In diabetic patients a positive correlation was found between plasma MDA levels and mean daily blood glucose (p < 0.01), plasma fructosamines (p < 0.001), HbA1 (p < 0.05) and plasma triglycerides (p < 0.05), while no significant correlation was shown between plasma malondialdehyde and total cholesterol. Malondialdehyde levels were followed-up for 7 days running (T1-T7) in five poorly controlled diabetics, treated with conventional insulin therapy. This group showed normalized plasma lipid peroxide values (0.486 +/- 0.13 mumol/l, T5, M +/- SEM) 72 h after the restoration of glycaemic control (145 +/- 25 mg/dl, T2, M +/- SEM). These results confirm the increase of lipid peroxidation during Type 2 diabetes. The correlation with the degree of metabolic imbalance suggests a possible role for lipid peroxidation in the occurrence of glucose-induced macromolecular changes.