The influence of indobufen (200 mg twice daily for 10 days) on platelet lipid peroxidation and phospholipid metabolism in diabetic patients was investigated. We have studied 18 patients with type I of diabetes mellitus. The duration of disease varied between 3 and 25 years (average 11 years). All patients showed symptoms of diabetic angiopathy. Investigations were carried out after restoring the carbohydrate balance. The following tests were performed before and after a 10 day treatment with indobufen: malondialdehyde, conjugated dienes and lipid hydroperoxides, phospholipids, fatty acids. We observed a significantly higher production of lipid peroxidation products in diabetic platelets compared to control platelets. After a 10 day treatment with indobufen the intensity of peroxidation lowered significantly. The phospholipid composition of diabetic platelets showed a significant increase in lysophosphoglycerides fraction compared to the control platelets. After the indobufen treatment a significant decrease in sphingomyelin content was observed. In other phospholipid fractions no significant differences were observed.