In the Paris Prospective Study, 7 484 men from the same professional group, 43 to 54 years old, have been followed for six and a half years. The incidence of coronary heart diseases (CHD) has been studied among those free of CHD at entry, in relation to parental history of CHD and high blood pressure (HBP), obtained by interview. In the presence of paternal history of CHD, the relative risk is 1.5, and in the presence of paternal history of HBP, the relative risk is 2 (after exclusion of the subjects with HBP). The following risk factors: age, blood pressure, cholesterol, diabetes, and cigarette smoking can explain but a small part of the higher risk. Maternal history of CHD or HBP is not associated with a higher risk. It is shown that the "classical" risk factors, paternal history of CHD, and paternal history of HPB all contribute independently to the risk of CHD. The simultaneous presence of the last two is associated with a relative risk of 3.