The effects of a high-carbohydrate, high-fiber diet and an olive-oil-rich diet on the distribution of cholesterol over the various lipoproteins, on serum apolipoproteins, and on the composition of HDL2 and HDL3 were studied under strict dietary control. Forty-eight healthy subjects first consumed a high-saturated-fat diet [proportion of energy, en%] (saturated fat 20 en%, total fat 38 en%) for 17 days. For the next 36 days, 24 subjects consumed a diet high in complex carbohydrates (monounsaturated fat 9 en%, total fat 22 en%) and the other 24 consumed a high-fat, olive-oil-rich diet (monounsaturated fat 24 en%, total fat 41 en%). The amounts of protein (12% to 14 en%), polyunsaturated fat (4 to 5 en%), and cholesterol (31 to 35 mg/MJ) were similar in all three diets. Serum cholesterol levels fell by 0.44 mmol/L in subjects consuming the carbohydrate diet and by 0.52 mmol/L for those receiving the olive-oil-rich diet. VLDL-cholesterol levels rose by 0.08 mmol/L in the carbohydrate group and fell by 0.08 mmol/L in the olive oil group (P < .05 for difference between test diets). HDL2 and LDL cholesterol levels fell to the same extent on both diets. HDL3 cholesterol fell by 0.09 mmol/L on the high-carbohydrate diet and increased by 0.01 mmol/L on the olive oil diet (P < .05). There was no change in the composition of HDL3, suggesting that the fall was due to a decrease in the total number of circulating particles. Thus, Keys formula, which predicts no change in serum cholesterol when carbohydrate is replaced by oleic acid, holds for LDL but not for VLDL and HDL, which change in opposite directions. Apolipoprotein (apo) A-I levels decreased by 102 mg/L in the carbohydrate group and rose by 26 mg/L in the olive oil group (P < .05). For apo B these values were 5 mg/L and −83 mg/L, respectively. The apo A-I to apo B ratio decreased by 0.12, or 10%, on the carbohydrate diet and increased by 0.19, or 16%, on the olive oil diet (P < .01). It is concluded that the lipoprotein risk profile for coronary heart disease was affected more favorably by the olive-oil-rich diet than by the diet high in complex carbohydrates.