Few controlled trials have studied cholesterol-lowering diets in premenopausal women. None has examined the cholesterol-lowering effect of a low-fat vegetarian diet, which, in other population groups, leads to marked reductions in serum cholesterol concentrations and, in combination with other life-style changes, a regression of atherosclerosis. We tested the hypothesis that a low-fat, vegetarian diet significantly reduces serum total and low-density lipoprotein (LDL) cholesterol concentrations in premenopausal women. In a crossover design, 35 women, aged 22 to 48, followed a low-fat vegetarian diet deriving approximately 10% of energy from fat for 2 menstrual cycles. For 2 additional cycles, they followed their customary diet while also taking a "supplement" (placebo) pill. Serum lipid concentrations were assessed at baseline and during each intervention phase. Mean serum LDL, high-density lipoprotein (HDL), and total cholesterol concentrations decreased 16. 9%, 16.5%, and 13.2%, respectively, from baseline to the intervention diet phase (p<0.001), whereas mean serum triacylglycerol concentration increased 18.7% (p<0.01). LDL/HDL ratio remained unchanged. Thus, in healthy premenopausal women, a low-fat vegetarian diet led to rapid and sizable reductions in serum total, LDL, and HDL cholesterol concentrations.