Middle-aged men who had had a myocardial infarction were compared with controls matched for social background, age, cigarette-smoking, blood pressure, and alcohol consumption. Serum cholesterol, triglycerides, very low density lipoprotein, low density lipoprotein, high density lipoprotein (HDL), HDL2 and HDL3 cholesterol, and serum apolipoproteins (apo) (a), AI, and B were measured. Discriminant analysis showed that the combination of these variables that best distinguished patients from controls was provided by apo AI and apo B and a knowledge of parental history of early cardiac death, the most discriminating single factor being apo B. No other variable contributed more than these. Apo (a), however, could be substituted for parental history, which had a major influence on the serum concentration of apo (a). Apo (a) concentration accounted for much of the familial predisposition to cardiac ischaemia. These findings may prove valuable in the clinical assessment of genetic susceptibility to myocardial infarction. They also support the hypothesis that serum apo (a) concentration is a genetic trait that predisposes to arterial thrombosis. Apo B emerged as the main lipoprotein determinant of coronary disease risk.