Estradiol fatty acid esters are potent lipophilic estrogens with antioxidant properties, transported by lipoproteins in blood. We investigated effects of oral and transdermal estradiol replacement therapy on concentrations of estradiol fatty acid esters in serum in postmenopausal women in a double-blind, randomized fashion. The first group (n = 9) received oral (2 mg/d); the second (n = 10), transdermal estradiol (patch delivering 50 microg/d); and the third group (n = 7), placebo treatment for 12 wk. After extraction of serum and separation of esterified estradiol from nonesterified estradiol, the concentration of saponified estradiol esters was measured by time-resolved fluoroimmunoassay. In the oral estradiol group, the median serum estradiol fatty acid ester concentration rose by 27%, from 77 to 98 pM (P = 0.028) but remained unchanged in the transdermal estradiol and placebo groups. The median concentrations of serum nonprotein-bound estradiol increased similarly in the oral and transdermal estradiol groups. The change in serum estradiol ester concentrations during treatment, but not that of nonesterified estradiol, correlated positively with enhanced forearm blood flow responses in vivo. These data raise the possibility that an increase in serum estradiol fatty acid esters may contribute to beneficial effects of oral estradiol treatment, compared with an equipotent dose of transdermal estradiol.