In 1980, serum total and high density lipoprotein (HDL) cholesterol concentrations and body mass index (weight/height2) were measured in groups of adult men from 13 countries. A standardized protocol was used for the drawing of blood and the preparation, storage, and transport of serum. Total and HDL cholesterol concentrations were determined in one laboratory to reduce methodological variability as much as possible. Mean serum total cholesterol concentrations were low in the groups from Africa (3.0-4.3 mmol/liter; 1 mmol/liter = 38.7 mg/100 ml), intermediate in the groups from Pakistan, the Philippines, Surinam, Hungary, Poland, and the Mediterranean countries (4.4-5.5 mmol/liter), and high in those from the Netherlands and Finland (5.6- 6.4 mmol/liter). Mean serum HDL cholesterol concentrations tended to be lower in the men from Africa, Asia, and Surinam (0.7-1.3 mmol/liter) than in those from Europe (1.1-1.5 mmol/liter), the highest values for both total and HDL cholesterol being found in the men from eastern Finland. The ratio of HDL cholesterol/total cholesterol varied from 0.15 to 0.32 and was on average slightly higher in the groups from Africa (0.26-0.32) than it was in the groups from Europe (0.20-0.28) and from Asia and Surinam (0.15-0.22). The body mass index was positively related to the concentration of total cholesterol and negatively related to the concentration of HDL cholesterol and the HDL cholesterol/total cholesterol ratio. The relationships between the concentration of HDL cholesterol and mortality from coronary heart disease within and between populations are discussed.