The effects of two strictly controlled diets, one rich in complex carbohydrates, the other rich in olive oil, on serum lipids were studied in healthy men and women. Serum cholesterol levels fell on average by 0.44 mmol/l in the carbohydrate group and 0.46 mmol/l in the olive oil group. HDL cholesterol levels fell by 0.19 mmol/l in the carbohydrate group and rose by 0.03 mmol/l in the olive oil group. Serum triglycerides rose by 0.19 mmol/l in the carbohydrate group and fell by 0.06 mmol/l in the olive oil group. The changes in both HDL and triglycerides were larger in men than in women. These results clearly show that the olive-oil-rich diet, unlike the complex-carbohydrate-rich diet, caused a specific fall in non-HDL cholesterol while leaving serum triglyceride levels virtually unchanged.