The incidence of ischemic heart disease shows a sharp rise after menopause. However, the effects of hormone replacement therapy (HRT) on cardiovascular disease are still controversial. Not only oxidative stress, but also inflammation has been suggested to play an important role in the pathogenesis of cardiovascular events. We compared the effects of HRT on endothelial function, cellular antioxidant system and inflammation between oral and transdermal administration in mild hypercholesterolemic postmenopausal women. Transdermal estradiol replacement was administrated to 12 patients (mean age 53 years) for 12 weeks, and oral conjugated equine estrogen was administrated to 12 patients (mean age 54 years) for 12 weeks. The flow-mediated endothelium-dependent dilation of the brachial artery, serum levels of thioredoxin as a marker of the cytoprotective antioxidant system, and high-sensitivity C-reactive protein (hs-CRP) were measured every 4 weeks. The flow-mediated vasodilation increased with HRT (oral, baseline 4.9 +/- 0.5, 4-week 8.9 +/- 0.7*, 8-week 9.9 +/- 0.6*, 12-week 9.4 +/- 0.7*; transdermal, 4.7 +/- 0.6, 8.3 +/- 0.7*, 9.1 +/- 0.8*, 8.9 +/- 0.9%*, * = p < 0.01 versus baseline). The thioredoxin levels decreased with HRT (oral, 26.1 +/- 7.2, 24.1 +/- 8.2, 22.1 +/- 7.8, 19.1 +/- 7.0*; transdermal, 26.9 +/- 7.4, 23.4 +/- 8.7, 21.1 +/- 7.9, 19.2 +/- 7.2 ng/ml*, * = p < 0.01 versus baseline). There were no differences in the variation of the flow-mediated vasodilation or thioredoxin concentrations between the 2 groups. The hs-CRP levels increased with oral HRT (0.32 +/- 0.12, 0.72 +/- 0.17*, 0.86 +/- 0.23*, 0.88 +/- 0.21 mg/dl*, * = p < 0.01 versus baseline), while transdermal HRT did not elicit any changes (0.35 +/- 0.15, 0.34 +/- 0.17, 0.38 +/- 0.20, 0.36 +/- 0.22 mg/dl). The differences of hs-CRP concentrations between the 2 groups analyzed by 2-way ANOVA were significant (p < 0.01). Oral HRT instigated inflammation, but transdermal did not. Both oral and transdermal HRT, however, improved endothelial function and decreased oxidative stress through affecting the cellular redox state. These differentials in the effects caused by the course of administration may affect the future cardiovascular events.