Serum/plasma sample pairs, collected both during (12-h) fasting and nonfasting from 44 healthy subjectes, were analyzed for total cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides. Values for total cholesterol and triglycerides averaged 2.1% and 5.2%, respectively, higher for serum than for plasma. Values for HDL cholesterol were not different. Triglyceride concentrations in nonfasting subjects were 40% higher than concentrations during fasting, but total cholesterol and HDL cholesterol were not significantly affected by fasting. These findings suggest that concentrations of HDL cholesterol may be directly compared in clinical and epidemiological settings without regard to whether serum or plasma was sampled or whether the subjects were fasting or nonfasting.