We fed 12 men with abnormally high insulin responses to a sucrose load and 12 controls normal diets containing 0, 7.5, or 15% of the calories as pure fructose for 5 weeks each in a crossover design. Purified wheat starch replaced the fructose in the 0 and 7.5% diets. The two groups were matched for age, height, and weight. At the beginning of the study and at the end of each of the three 5-week periods, plasma responses to a sucrose load (2 gm/kg body weight) were measured. Initially there were no significant differences in the plasma fructose, uric acid, or inorganic phosphorus responses of the two groups. Plasma fructose responses to a sucrose load were significantly higher after the men consumed the 7.5 and 15% fructose diets than after the 0% diet. Uric acid responses tended to be greater in the hyperinsulinemic men than in controls and increased as the levels of fructose in the diet increased. The inorganic phosphorus levels after a sucrose load were higher in the hyperinsulinemic men than in controls when they consumed the 7.5 and 15% fructose diets. These results indicate that moderate levels of dietary fructose can affect plasma fructose, uric acid, and inorganic phosphorus levels, especially in hyperinsulinemic men.