The short-term effects on plasma lipoprotein lipids of substituting meat and dairy protein for carbohydrate in the diets of 10 free-living moderately hypercholesterolemic human subjects (four men, six women) were studied under closely supervised dietary control during the consumption of constant, low intakes of fat and cholesterol and the maintenance of stable body weight as well as constant fiber consumption. Subjects were randomly allocated to either the high or low protein diets (mean, 23% v 11% of energy as protein, 24% as fat, and 53% v 65% as carbohydrate) and then switched to the other diet for another 4 to 5 weeks. Mean fasting plasma high-density lipoprotein cholesterol (HDL-C) was significantly higher by 12% +/- 4% (0.97 +/- 0.08 v 0.89 +/- 0.08 mmol/L, P less than .01), whereas mean total cholesterol (TC) was lower by 6.5% +/- 1.3% (5.7 +/- 0.3 v 6.1 +/- 0.3 mmol/L, P less than .001), mean low-density lipoprotein-cholesterol (LDL-C) lower by 6.4% +/- 2.0% (4.5 +/- 0.2 v 4.8 +/- 0.2 mmol/L, P less than .02), mean total triglycerides (TG) lower by 23% +/- 5% (1.7 +/- 0.1 v 2.4 +/- 0.3 mmol/L, P less than .02), and mean high versus low protein diet. Mean values for LDL-C were significantly lower during weeks 3 to 5 of the high protein diet than during either weeks 1 to 5 or weeks 1 to 2 of the high protein diet (4.3 +/- 0.3, 4.5 +/- 0.2, and 4.7 +/- 0.3 mmol/L, respectively, P less than .05) and 11% +/- 3% lower than on low protein diet, P less than .005. The ratio of plasma LDL-C to HDL-C was consistently lower by 17% +/- 3% during the high versus low protein diet (4.9 +/- 0.5 v 5.8 +/- 0.5, P less than .001). Lowering plasma TC and LDL-C and total TG and VLDL-TG and increasing HDL-C by chronic isocaloric substitution of dietary for carbohydrate may enhance the cardiovascular risk reduction obtained by restriction of dietary fat and cholesterol.