The importance of a positive family history as a primary risk factor for coronary heart disease was examined in a case history study. Of 792 consecutive male patients aged under 60 years who survived a first episode of unstable angina or myocardial infarction, 326 had a negative family history, 298 had a positive history, and in 168 a family history could not be established with certainty. There was no significant difference in the distribution of the three primary coronary risk factors--cigarette smoking, hypertension, and hypercholesterolaemia--between those with and those without a positive family history. The 133 subjects with a positive family history of premature coronary heart disease (occurrence in near relatives under 60 years) were significantly younger than those with a negative family history. It is concluded that there is little evidence to confirm a positive family history as an important independent risk factor for coronary heart disease, although there may be familial aggregation of subjects with a high susceptibility to the effects of the three primary risk factors, cigarette smoking, hypertension, and hyperlipidaemia.