Objective: To assess the antioxidant/non-antioxidant effects of a hydroxytyrosol (HT)-rich phenolic extract from olive mill wastewaters administered with a breakfast.Design, setting and subjects: Five type I diabetic patients received 25 mg of HT the first day and 12.5 mg/day the following 3 days. Blood sampling was carried out at T-0 ( baseline) and T-4d just before the breakfast+HT administration and at time points 1, 2, 3 and 4 h after T-0. Urines (24-h) were collected from T-0 to T-4d. Baseline HbA1c was generally inferior to 10%, glycemia was within the range 6 - 24 mmol/ l, whereas total cholesterol, HDL-chol and triglycerides were normal.Results: The major finding was the 46% decrease in the serum TXB2 production after blood clotting at T-4d. Plasma vitamin A, E, β- carotene were not changed. Vitamin C tended to increase ( P = 0.075). Plasma antioxidant capacity was enhanced at T-0+1 h only, whereas its main determinants ( albumin, bilirubin, uric acid) were not modified. Urinary 8-isoPGF(2α) levels were highly variable and were not affected significantly by HT administration.Conclusion: The major effect of HT accounts for an antiaggregating platelet action, leading to a possible prevention of thrombotic and microthrombotic processes.