The most stringent dietary recommendations of the National Cholesterol Education Program (NCEP) are to limit fat intake to <30% of calories, saturated fat intake to <7% of calories, and cholesterol intake to <200 mg/d (Step 2 diet). There is debate as to whether the remaining fat in the diet should be relatively high in monounsaturated or poly unsaturated fatty acids. We examined this issue by testing the effects of diets meeting the aforementioned guidelines that were enriched in three different vegetable oils on plasma lipids in the fasting and postprandial states in a clinically relevant population. Female and male subjects (n=15, mean age, 61 years) with low-density lipoprotein cholesterol (LDL-C) concentrations >130 mg/dL were studied under strictly controlled conditions. Subjects were first placed on a diet similar to that currently consumed in the United States to stabilize plasma lipids with respect to identical fat and cholesterol intakes. The subjects then received diets meeting NCEP Step 2 criteria in which two thirds of the fat calories were given either as canola, corn, or olive oil in a randomized, double-blinded fashion for 32 days each. Plasma cholesterol concentrations declined after consumption of diets enriched in all the test oils; however, the declines were significantly greater for the canola (12%) and corn (13%) than for the olive (7%) oil-enriched diet Mean plasma LDL-C concentrations declined after consumption of diets enriched in all the test oils (16%, 17%, and 13% for canola, corn, and olive oil, respectively), and the magnitude of the declines was statistically indistinguishable among the test oils. Mean plasma high-density lipoprotein cholesterol (HDL-C) concentrations declined after consumption of the baseline diet, and these declines were significant for the canola (7%) and corn (9%) oil-enriched diets. Changes in LDL apolipoprotein (apo)B concentrations paralleled those of LDL-C. Switching from the baseline to the vegetable oil-enriched diets had no significant effect on plasma triglyceride, apoA-I, and lipoprotein(a) concentrations or the total cholesterol to HDL-C ratio. LDL apoB to apoA-I ratios were significantly reduced when the subjects consumed the vegetable oil-enriched diets. Differences similar to those observed in the fasting state were observed in the postprandial state. The major finding of this study is that significant reductions in LDL-C and apoB levels can be achieved in middle-aged and elderly women and men with initial LDL-C levels > 130 mg/dL by reducing dietary saturated fat and cholesterol intake and by incorporating vegetable oils rich in either monounsaturated fatty acids (canola and olive oil) or polyunsaturated fatty acids (corn oil) as part of an NCEP Step 2 diet Although differential effects were seen after the consumption of the three different oil-enriched diets in some plasma lipid measures, none of these oils had a significant advantage in terms of altering the overall lipoprotein profile.