Two levels of dietary carbohydrate (40% and 60% of calories) were incorporated into typical U.S. diets and fed for 10 days each to 11 healthy volunteers. Fasting blood samples were drawn on days 8, 9, and 10 of each dietary period and analyzed for glucose, insulin, cholesterol, triglyceride (TG) and high density lipoprotein (HDL)-cholesterol concentrations. In addition, plasma glucose, insulin and TG concentrations were determined before, and for 3 hr after the noon meal on days 8 and 10. No differences were observed in fasting plasma glucose, insulin or cholesterol concentrations. However, fasting plasma TG levels were significantly elevated on the 60% carbohydrate diet, and HDL-cholesterol concentrations were significantly decreased. Furthermore, the plasma insulin and triglyceride responses to the meal tolerance test during the 60% carbohydrate dietary period were significantly elevated. These results indicate that high-carbohydrate diets lead to changes in insulin, TG, and HDL-cholesterol concentrations which have been associated with an increase in incidence of coronary artery disease.