The purpose of these studies was to find large groups with significantly less coronary atherosclerosis than New Yorkers and to investigate the possible reasons for these differences. Direct comparison of hearts and measurements of coronary artery wall thickness, autopsy series, and clinical diagnoses of outpatients and hospital admissions revealed that the amount of coronary atherosclerosis and the number of myocardial infarcts is significantly less in East and West Africans compared to New Yorkers matched for age and sex. The factors producing these differences are apparently operative in childhood. East Africans were found to have a shorter blood clot-lysis time, fewer venous (and arterial) thromboemboli and lower serum lipid levels, with a lower relative percentage of serum linoleates, than age-matched New Yorkers.