Estrogen decreases low density lipoprotein (LDL) particle size, and smaller LDL particles are associated with coronary atherosclerosis. To understand the metabolic basis for this change, we studied the effect of oral 17 beta-estradiol (2 mg/day) on apolipoprotein B-100 (apoB) metabolism, in eight healthy postmenopausal women. The study was a randomized, double blinded, placebo-controlled, cross-over trial with intervention sequences of 6 weeks each. ApoB in very low density lipoprotein, intermediate density lipoprotein, and LDL subclasses was endogenously labeled with [D3]L-leucine, and metabolic rates were calculated by computer modeling. The overall effect of oral estrogen therapy on apoB metabolism was to accelerate the fractional catabolic rates of all particles studied and production rates of all except IDL. For light LDL (density = 1.019-1.036 g/mL), estrogen increased the mean fractional catabolic rate by 63% from 0.59 to 0.96 pools/day (P = 0.02), whereas the production rate increased by a lesser amount (42%) from 575 to 817 mg/day (P = 0.10). These metabolic changes reduced light LDL cholesterol and apoB concentrations by 26% (P = 0.005) and 19% (P = 0.03), respectively. In contrast, dense LDL (density = 1.036-1.063 g/mL) cholesterol and apoB concentrations were unchanged by the intervention, as both the apoB fractional catabolic rate and production rate were significantly increased by similar amounts, 39% (from 0.41 to 0.57 pools/day, P = 0.01) and 38% (from 434 to 601 mg/day; P = 0.003), respectively. Estrogen decreased the predominant LDL peak particle size from 273 to 268 A (P = 0.04). Thus, estrogen therapy increases the clearance of both light and dense LDL, counteracting increases in production rates. The reduced plasma residence times of light and dense LDL both may be antiatherogenic, even though, for dense LDL, the concentration did not change.