A real difference in serum cholesterol levels exists between male blood donors in New Orleans and Guatemala ( P < 0.05). The mean level is moderately higher in New Orleans men than in Guatemalan men (179 vs 169 mg/100 ml). Levels of serum cholesterol in donors parallel arterial atherosclerotic involvement in autopsied cases from the same geographic locations. The difference in cholesterol levels is not as much as might be expected considering the large difference in per cent surface involvement with atherosclerotic lesions. A real difference in mean serum triglyceride levels exists between male blood donors in New Orleans and Guatemala ( P < 0.01). Triglycerides are lower for New Orleans men than for Guatemalan men (65 vs 113 mg/100 ml). Levels of serum triglycerides do not parallel arterial atherosclerotic involvement in these two geographic locations. Smoking habits are different among blood donors from New Orleans and Guatemala. The New Orleans men smoke at an average greater rate than Guatemalan men (17 vs 7 cigarettes per day). Systolic blood pressure is higher for New Orleans donors than for Guatemalan donors (135 vs 120 mm Hg; P < 0.01). Combined risk factors for CHD are found more frequently in the New Orleans donors than in the Guatemalan donors.