F₂-isoprostanes are bioactive prostaglandin (PG) -like compounds that are produced from arachidonic acid through a nonenzymatic process of lipid peroxidation catalyzed by oxygen free-radicals. 8-Epi-PGF_2α may amplify the platelet response to agonists, circulates in plasma, and is excreted in urine. We examined the hypothesis that the formation of 8-epi-PGF_2α is altered in patients with hypercholesterolemia and contributes to platelet activation in this setting. Urine samples were obtained from 40 hypercholesterolemic patients and 40 age- and sex-matched control subjects for measurement of immunoreactive 8-epi-PGF_2α. Urinary excretion of 11-dehydro-thromboxane (TX) B₂, a major metabolite of TXA₂, was measured as an in vivo index of platelet activation. Low-dose aspirin, indobufen, and vitamin E were used to investigate the mechanism of formation and effects of 8-epi-PGF_2α on platelet activation. Urinary 8-epi-PGF_2α was significantly (P=.0001) higher in hypercholesterolemic patients than in control subjects: 473±305 versus 205±95 pg/mg creatinine. Its rate of excretion was inversely related to the vitamin E content of LDL and showed a positive correlation with urinary 11-dehydro-TXB₂. Urinary 8-epi-PGF_2α was unchanged after 2-week dosing with aspirin and indobufen despite complete suppression of TX metabolite excretion. Vitamin E supplementation was associated with dose-dependent reductions in both urinary 8-epi-PGF_2α and 11-dehydro-TXB₂ by 34% to 36% and 47% to 58% at 100 and 600 mg daily, respectively. We conclude that the in vivo formation of the F₂-isoprostane 8-epi-PGF_2α is enhanced in the vast majority of patients with hypercholesterolemia. This provides an aspirin-insensitive mechanism possibly linking lipid peroxidation to amplification of platelet activation in the setting of hypercholesterolemia. Dose-dependent suppression of enhanced 8-epi-PGF_2α formation by vitamin E supplementation may contribute to the beneficial effects of antioxidant treatment.