The effect of the fatty acid composition of reduced-fat diets on the in vitro oxidation of LDL was examined in 14 moderately hypercholesterolemic [low density lipoprotein (LDL) > 3.36 mmol/L] postmenopausal female and male subjects (age 44-78 y). Each subject consumed each of five reduced-fat diets [30 energy percent (E%) fat, 17 E% protein and 53 E% carbohydrate] enriched in beef tallow, canola oil, corn oil, olive oil or rice bran oil (20 E%) for 32-d periods. In vitro oxidation of LDL was assessed by incubating LDL with hemin and hydrogen peroxide, and measuring the time required for the reaction to reach maximum velocity (lag time). LDL lag times were 93.2 +/- 25.8, 95.9 +/- 26.4, 104.2 +/- 32.7, 108.0 +/- 26.6 and 113.1 +/- 24.0 min for corn oil, beef tallow, rice bran oil, canola oil and olive oil periods, respectively. When the data from all dietary phases were pooled, LDL alpha-tocopherol level (r = 0.30, P = 0.01) and plasma 18:1/18:2 ratio (r = 0.22, P = 0.08) were positively related to resistance of LDL to oxidation. Differences induced by the dietary perturbations in LDL content of beta-cryptoxanthin, lutein/zeaxanthin, lycopene, alpha-carotene or beta-carotene, and LDL particle size were not related to resistance of LDL to oxidation. In conclusion, in middle-aged and elderly moderately hypercholesterolemic subjects, the consumption of reduced-fat diets enriched in animal fat or vegetable oils with a relatively wide range of fatty acid profiles did not alter the in vitro susceptibility of LDL to oxidation. The advantages of reducing the saturated fat content of the diet were reflected in lower total and LDL cholesterol levels.