Increased free radical activity may contribute to thrombosis via effects on platelet aggregation and the prostanoid balance. To investigate this further we studied 15 Type 1 diabetic patients with retinopathy, matched with uncomplicated Type 1 patients for age, duration of diabetes and HbA1, together with matched healthy non-diabetic control subjects. The oxidative effects of free radicals as total diene conjugates and lipid peroxides were measured, together with redox status extracellularly as plasma albumin-thiols and intracellularly as erythrocyte superoxide dismutase activity. Platelet count, aggregation of platelets in whole blood to collagen, thromboxane B2, and prostacyclin stimulating factor (PGI2SF) were also assessed. Free radicals measured as lipid peroxides were significantly higher (9.6 (8.1-11.6) mumol l-1 (median and interquartile range) in diabetic patients with retinopathy than in control subjects (8.1 (7.4-9.2) mumol l-1; p less than 0.05). There were also significant reductions in redox status both extracellularly as plasma albumin thiols (408 (383-473) vs 490 (456-517) mumol l-1, p less than 0.001) and intracellularly as erythrocyte superoxide dismutase activity (34 (27-41) vs 44 (36-51) g l-1, p less than 0.05) between patients with retinopathy and control subjects. Platelet counts were increased in diabetic patients with retinopathy (p less than 0.05), as was collagen-induced platelet aggregation (p less than 0.01). Prostacyclin stimulating factor was reduced in patients with retinopathy (p less than 0.05) and correlated within the plasma with lipid peroxides (r = -0.53, p less than 0.04) and albumin thiols (r = 0.64, p less than 0.01). The results suggest that diabetic patients, particularly with retinopathy, are under oxidative stress and have an increased thrombotic tendency with increased platelet reactivity and a reduction in prostacyclin stimulating factor.