For 6 weeks, 10 men and nine women aged 35 to 55 consumed each of two diets in a cross-over design. The diets were comprised of identical natural foods with 30% of the calories as either sucrose or cooked wheat starch. Carbohydrate, fat and protein supplied 43, 42, and 15% of the calories, respectively. Of the calories 10% were eaten at breakfast (7:00 to 8:30 AM) and 90% at dinner (4:30 to 6:30 PM). Initial body weights were essentially maintained. Total serum lipids, triglycerides, and total cholesterol levels were significantly higher when the subjects consumed the sucrose diet than when they consumed the starch diet. Increases associated with the sucrose diet were greatest for triglycerides (33.0%). In a subgroup of nine subjects with triglyceride levels above the normal range, sucrose feeding increased triglyceride levels 45.2%. Triglycerides and pre-beta lipoproteins were significantly higher in males than in females. Pre-beta lipoproteins were 32% higher when the subjects consumed sucrose than when they consumed starch. For alpha and beta lipoproteins, small, nonsignificant increases were associated with sucrose feeding. Serum free fatty acids were not affected by diet. These results indicate that the consumption of sucrose can increase blood lipids that are considered to be risk factors in heart disease and that males and carbohydrate-sensitive individuals may be more susceptible than others to the effects of sucrose.