A consistent predictor of a woman's risk for breast cancer is a family history of the disease. Most studies of family history and breast cancer have used the number of affected relatives in the family to calculate relative risk, but they have not considered the heterogeneity of the familial risk for breast cancer in a systematic way. With the use of data from a large prospective mortality study of US adults, the authors compared simple classification of family history of breast cancer (yes/no) to the method of using a quantitative family history score method, which takes into account the effects of family structure, age, and birth cohort as predictors of breast cancer mortality. After 9 years of follow-up, 1,428 cases of fatal breast cancer were observed among 453,073 women with complete information on number and age of siblings and family history. With the use of the family history score, about one-third of women with a positive family history of breast cancer were at no higher risk for breast cancer mortality than those without a family history of the disease. As a quantitative measure of relative risk for each family, family history score gave a better fit to the data, and it provided an incremental improvement of predictive accuracy of developing fatal breast cancer. Family history score can also be used as a categorical variable to stratify families. This allows researchers to focus on which risk groups would benefit from conducting further genetic analysis and to test the effects of genetic factors, environmental exposure, and gene-environment interactions on the etiology of the development of breast cancer.