Low-fat diets, in which carbohydrates replace some of the fat, decrease serum cholesterol. This decrease is due to decreases in LDL-cholesterol but in part to possibly harmful decreases in HDL-cholesterol. High-oil diets, in which oils rich in monounsaturated fat replace some of the saturated fat, decrease serum cholesterol mainly through LDL-cholesterol. We used these two diets to investigate whether a change in HDL-cholesterol would change flow-mediated vasodilation, a marker of endothelial function. We fed thirty-two healthy volunteers two controlled diets in a 2×3·5 weeks' randomised cross-over design to eliminate variation in changes due to differences between subjects. The low-fat diet contained 59·7 % energy (en%) as carbohydrates and 25·7 en% as fat (7·8 en% as monounsaturates); the oil-rich diet contained 37·8 en% as carbohydrates and 44·4 en% as fat (19·3 en% as monounsaturates). Average (SD) SERUM HDL-CHOLESTEROL AFTER THE LOW-FAT DIET WAS 0·21 (sd 0·12) mmol/l (8·1 mg/dl) lower than after the oil-rich diet. Serum triacylglycerols were 0·22 (sd 0·28) mmol/l (19·5 mg/dl) higher after the low-fat diet than after the oil-rich diet. Serum LDL and homocysteine concentrations remained stable. Flow-mediated vasodilation was 4·8 (SD 2·9) after the low-fat diet and 4·1 (SD 2·7) after the oil-rich diet (difference 0·7 %; 95 % CI -0·6, 1·9). Thus, although the low-fat diet produced a lower HDL-cholesterol than the high-oil diet, flow-mediated vasodilation, an early marker of cardiovascular disease, was not impaired.