The relative risk of developing future coronary heart disease (CHD) or hypertension between positive and negative family history families is compared for different definitions of a positive family history when applied to life-table data for 94,292 persons. Having two or more first degree relatives with CHD identifies 8% of the population with relative risks of 3.3-5.9 for CHD before age 50. A quantitative family history score (FHS) compares the family's age and sex specific disease incidence to that expected in the general population and predicts future disease incidence in unaffected family members slightly better (relative risks = 3.4-6.9 for CHD before age 50). Using only one affected relative, even if affected at an early age (less than 55 years old) does not discriminate low and high risk families as well (relative risks = 1.4-3.9 for CHD before age 50). Similar results were obtained for family history of hypertension. There is an increase in future disease incidence for all ages with increasing FHS values (p less than 0.0001), which can be used as a continuous or categorical variable in analysis where family history is associated with a particular variable under study. These results provide a rational basis for choosing and applying specific definitions of a positive family history of coronary disease or hypertension in clinical, epidemiologic and genetic studies.