ObjectiveTo investigate the effects of estrogen on the size of low-density lipoprotein (LDL) particles in postmenopausal women.MethodsWe treated 20 postmenopausal women with 0.625 mg of conjugated equine estrogen daily for 3 months and measured the plasma levels of total cholesterol, triglyceride, high-density lipoprotein (HDL), and apolipoproteins A-I, A-II, and B before and after therapy. We also analyzed concentrations of LDL cholesterol and LDL apolipoprotein B. The diameter of LDL particles was determined by gradient gel electrophoresis.ResultsEstrogen caused significant decreases in the plasma levels of total cholesterol and apolipoprotein B and significant increases in the plasma levels of triglyceride, HDL cholesterol, and apolipoprotein A-I and A-II. Mean levels of LDL cholesterol and LDL apolipoprotein B were reduced significantly (by 16.31%,P< .001, and 16.91%,P< .001, respectively) after estrogen treatment. Estrogen also significantly reduced the size of LDL particles, from 25.74 ± 0.66 (mean ± standard deviation) to 24.95 ± 0.78 nm (P< .001). The LDL particle diameter correlated negatively with the plasma level of triglyceride (pre-treatment:r= 0.87,P< .001; post-treatment:r= 0.88,P< .001). Estrogen significantly increased the prevalence of LDL subclass pattern B, from 30 to 65% (P< .03).ConclusionEstrogen affects lipid metabolism favorably by reducing the plasma concentration of LDL particles. Estrogen-induced increase in the plasma level of triglyceride appears to reduce the size of LDL particles.