Lipid peroxides are thought to be formed by free radicals and may play an important role in the development of atheromatous vascular disease. We have investigated the relationship between lipids, lipoproteins, coagulation factors, and lipid peroxides (measured as thiobarbituric acid reacting species (TBARS) in Type 2 diabetic patients with macrovascular disease. Eighteen diabetic and 20 non-diabetic subjects with clinical evidence of ischaemic heart disease and/or peripheral vascular disease were investigated, together with 28 healthy subjects without evidence of vascular disease. TBARS concentrations in non-diabetic (mean 5.0 (95% Cl 4.5-5.7) mumol l-1) and diabetic groups (5.6 (5.1-6.0) mumol l-1) with macrovascular disease were not significantly different although values were higher in both groups of patients with vascular disease by comparison with control subjects (2.7 (2.4-3.1) mumol l-1, p less than 0.001). Significant univariate correlations between TBARS concentrations and measures of blood glucose control (fructosamine, blood glucose and HbA1) were found for all 66 subjects (r = 0.35-0.42, p less than 0.01-p less than 0.001), although no independent association between these parameters and TBARS was demonstrated in multiple regression analysis.(ABSTRACT TRUNCATED AT 250 WORDS)