Serum total and HDL cholesterol concentrations were measured in schoolboy populations from 16 countries. Total and HDL cholesterol concentrations were determined in one laboratory to reduce methodological variability as much as possible. A standardized protocol was used for the drawing of blood, preparation, storage and transport of serum. Mean serum total cholesterol concentrations were relatively low in the less developed countries and relatively high in the more developed ones. Mean serum HDL cholesterol concentrations showed a distribution similar to that of mean total cholesterol: relatively low levels in the less developed countries and relatively high levels in the more developed ones. Statistically significant relationships were found between serum total and HDL cholesterol between individuals, between populations and within populations. The relationship between HDL cholesterol concentrations and diet is discussed.