The effect of dietary composition on concentrations of postprandial lipoproteins was studied in eight sulfonylurea-treated patients with noninsulin dependent diabetes mellitus. Two diets were consumed by each patient for 2 weeks in random order, one contained (as percent of total calories) 15% protein, 40% fat, and 45% carbohydrate (CHO), whereas the other consisted of 15% protein, 25% fat, and 60% CHO. At the end of each dietary period, patients were given Vitamin A (60,000 U/m2) with their noon meal, and the concentration of triglyceride (TG) and retinyl esters in plasma and two lipoprotein fractions (Sf > 400 and Sf 20-400) determined over the next 12 h. The results indicated that both postprandial TG and retinyl ester concentrations were higher in plasma (Sf > 400, and Sf 20-400 lipoproteins), when patients ate the 25% fat/60% CHO diet. Thus, replacing saturated fat with CHO accentuates the magnitude of postprandial lipemia. Since TG-rich lipoproteins may be atherogenic, appropriate dietary advice for patients with type 2 diabetes may deserve reappraisal.