We have studied the effects of olive oil on serum lipids in a dietary experiment with 48 healthy adult Dutch volunteers and in 76 boys 8-10 years of age from rural Crete. The dietary trial was carried out with normolipidaemic subjects, and it compared the effects of an olive-oil-rich and a carbohydrate-rich, high-fibre diet on serum lipids. Replacement of 12% of energy (en%) from saturated fatty acids by either monounsaturates or complex carbohydrates caused the same fall in serum total cholesterol of 0.45 mmol 1(-1). HDL cholesterol levels fell by 0.19 mmol 1(-1) on the carbohydrate-rich diet, but remained virtually unchanged on the olive-oil-rich diet. This was in agreement with our hypothesis that the total amount of fat in the diet is a determinant of HDL cholesterol. The diet of the Cretan boys contained 10 en% saturated fat, but 27 en% monounsaturated fat due to the liberal use of olive oil. Serum-lipid levels of the Cretan boys were not different from those of their counterparts from western European Countries. The mean body mass index of the Cretan boys was about 2 kg m-2 higher than those of boys in the other countries, which might have confounded comparisons. Whatever the explanation, the present-day lifestyle and diet in Crete do not produce a particularly favourable HDL to LDL cholesterol ratio in the blood, even though olive oil intake is high.