The present paper compares the effects of two monounsaturated oils, extra virgin olive oil (EVOO) and high-oleic acid sunflower oil (HOSO), on serum and LDL peroxides, eicosanoid production and the thrombogenic ratio (thromboxane (TX) B2:6-keto-prostaglandin F1α) in fourteen non-obese post-menopausal women. The subjects, mean age 63 (SD 11) years, were assigned to two consecutive oleic acid-rich 28 d dietary periods. EVOO and HOSO represented 62 % of the total lipid intake and were used as the only culinary fat during the first and second dietary periods respectively. Serum peroxides, plasma α-tocopherol and TXB2 levels in stimulated platelet-rich plasma (PRP-TXB2) were significantly higher (P<0.01, P<0.001, and P<0.05 respectively) after the HOSO diet than after the EVOO diet. The relationship between the serum cholesterol level (<6.21 mmol/l or ≥6.21 mmol/l) and the type of dietary oil on eicosanoids, peroxides and α-tocopherol were evaluated by two-way ANOVA. Dietary oil significantly affected (P<0.05) the PRP-TXB2 level, whereas serum and LDL peroxides were significantly affected (P<0.001 and P<0.01 respectively) by the serum cholesterol level. The plasma α-tocopherol level was significantly affected by the serum cholesterol level and the type of dietary oil (both P<0.001). No significant relationships were found between serum cholesterol levels, serum peroxide or LDL peroxide levels, plasma α-tocopherol concentrations or α-tocopherol intakes with eicosanoid production or the thrombogenic ratio due to dietary changes. However, in spite of their higher α-tocopherol levels, hypercholesterolaemic subjects showed increased peroxidation in serum and LDL in comparison with normocholesterolaemic subjects on the HOSO diet in comparison with the EVOO diet. These findings suggest that differences in the type of minor compounds, as well as in the concentration of linoleic acid, in both these monounsaturated oils may play an important role in modulating eicosanoid production and lipoprotein peroxidation when they constitute a large proportion of the diet of post-menopausal women.