Serum samples were collected from 7- and 8-year-old boys in 16 countries with different rates of coronary heart disease (CHD) mortality. Both serum total and high-density lipoprotein (HDL) cholesterols were lower in developing countries than in affluent countries. The mean of the HDL cholesterol/total cholesterol ratio varied within narrow limits: 0.24 – 0.27 in Asian countries, 0.30 – 0.36 in Africa, and 0.30 – 0.37 in the USA and Europe. Thus both total and HDL cholesterol increased under the influence of a more affluent diet. This conclusion is supported by results from controlled trials. Student volunteers received all their food from us for periods of 8 to 16 weeks. Diets were prepared from regular foodstuffs, in such a way that they differed in one nutrient only. Dietary composition was confirmed by double portion analysis. A moderate fat diet with a high P/S ratio was compared with a low-fat, low-PUFA diet and with two high-fat diets having a high or low P/S ratio. Both low-fat and high P/S diets effectively lowered total serum cholesterol. However, HDL was depressed by the low-fat diet in comparison with diets with a higher fat content, whether high or low in polyunsaturates. This difference in HDL persisted for at least 3 months. This suggests that "Western" diets, usually high in fat, elevate both total and HDL cholesterol concentrations in children and young adults.