The effect on plasma lipoproteins of exchanging fat type within currently recommended reduced-fat diets was studied in a free-living group of 19 men and 20 women who consumed both a polyunsaturated fat-enriched diet and a monounsaturated fat-enriched diet, each for a 12-week period, with saturated fat and cholesterol held constant. Mean plasma concentrations of low-density lipoprotein cholesterol and low-density lipoprotein total mass (analytic ultracentrifuge), and high-density lipoprotein (HDL) cholesterol and HDL total mass, did not change significantly on exchanging fat type. However, HDL2 cholesterol concentration was 50% higher and HDL3 cholesterol concentration was 7% lower for polyunsaturated compared with monounsaturated fat. Mean total mass of HDL2 was also 23.5% higher and concentration of apolipoprotein B was 5.4% lower on transfer to the polyunsaturated fat diet. Contrary to frequent assertions, we find no advantage with respect to plasma HDL concentrations in using predominantly monounsaturated rather than polyunsaturated fats in subjects who consumed reduced-fat, solid-food diets.