Cholesterol-lowering effects of polyunsaturated and monounsaturated fatty acids were compared as they were varied in a reciprocal dose-dependent fashion in the context of a National Cholesterol Education Program (NCEP) Step 1 diet. The study population comprised 63 moderately hypercholesterolemic African American and white men and women. After a 6-wk baseline diet containing 37% of energy from total fat and 15% from saturated fat, participants consumed four diets for 6 wk each, in random order, containing 10% of energy as saturated fatty acids; 3%, 6%, 10%, and 14% of energy as polyunsaturated fatty acids; and 17%, 14%, 10%, and 6% of energy as monounsaturated fatty acids. Dietary cholesterol, fiber, plant sterol, and squalene contents were constant with all four diets. There was a progressive decrease in total (P = 0.028) and low-density-lipoprotein cholesterol (P = 0.184) across the four diets, with the greatest decrease observed in the diet with the highest content of polyunsaturated fatty acids; a small but significant decrease in high-density-lipoprotein (HDL) cholesterol that did not show a trend between the polyunsaturated and monounsaturated diets; and a trend between the four diets in triacylglycerol elevations (P = 0.029), with the smallest increment occurring in the diets highest in polyunsaturates. The magnitude of the cholesterol-lowering response was greater in those with higher baseline cholesterol and less in those who were more obese. The dietary response was similar in both ethnic groups and in both sexes. In conclusion, in an NCEP Step 1 diet containing 30% total fat, with all other known cholesterol-influencing dietary factors held constant, the substitution of polyunsaturated fatty acid for monounsaturated fatty acid from 3% to 14% resulted in a progressive decline in total cholesterol and less triacylglycerol elevations, without effect on HDL cholesterol.