Abstract Objective: To investigate the effects of estrogen on lipid transfer reactions and lipid composition of low-density lipoprotein (LDL) particles in postmenopausal women. Methods: Twelve postmenopausal women were treated with conjugated equine estrogen, 0.625 mg daily, for 3 months. Plasma concentrations of total cholesterol, triglyceride, and high-density lipoprotein (HDL) cholesterol were measured before and after therapy. We also determined the amount of total, free, and esterified cholesterol, triglyceride, and apolipoprotein B in LDL. To evaluate lipid transfer reactions, plasma samples were incubated at 37C for 24 hours, and replacement of cholesteryl ester by triglyceride in LDL particles was analyzed. Cholesterol and triglyceride concentrations were measured enzymatically. Apolipoprotein B concentrations were determined by an immunoturbidimetric assay. Results: Estrogen significantly reduced the plasma levels of total cholesterol and significantly increased those of triglyceride and HDL cholesterol. The ratio of cholesteryl ester to apolipoprotein B was reduced significantly, whereas the ratio of triglyceride to apolipoprotein B increased significantly after estrogen treatment. Both before and after estrogen treatment, incubation of plasma induced a significant increase in the ratio of LDL-triglyceride to apolipoprotein B with a concomitant decrease in the ratio of LDL–cholesteryl ester to apolipoprotein B. Incubation-induced changes in these ratios were significantly enhanced by estrogen therapy. The plasma concentration of triglyceride was correlated positively with incubation-induced changes in the ratio of LDL-triglyceride to apolipoprotein B ( r = .83, P < .001) and correlated negatively with changes in the ratio of LDL–cholesteryl ester to apolipoprotein B ( r = −.61, P < .01). Conclusion: Estrogen-induced increase in the plasma level of triglyceride may enhance lipid transfer reactions, resulting in triglyceride-rich and cholesteryl ester-poor LDL particles.