Summary. Four hundred and eighty-six patients with myocardial infarction of both sexes comprising 213 Chinese, 53 Malays, 199 Indians, and 21 Europeans, resident in Singapore were investigated for differences in relative incidence, and for the ABO, Rh, Lea, and Xga blood groups, G6PD deficiency, and haemoglobin types. Relative incidence of myocardial infarction was appreciably higher among Indians than Chinese and Malays in all age groups, more so in younger people. There was no significant difference in the distribution of the genetic markers between patient and control series in either Chinese, Malays, or Indians.Results of twin studies are suggestive of some genetic factor involved in the causation of coronary thrombosis (Hauge, 1969) (Danaraj et al, 1959;Toh, Low, and Tan, 1970;Barnes, 1972). The multiracial population of Singapore with a population of about two million made up of approximately 76% Chinese, 15% Malays, and 7% Indians seemed to us therefore to offer special opportunities to study some of the ethnic and genetic factors involved in the development of myocardial infarction by analysing the incidence of the disorder in different ethnic groups and looking for possible association of myocardial infarction with the ABO, Rh, Lea, Xga blood groups, G6PD deficiency, and abnormal haemoglobins.