Recent linkage analyses demonstrate the strength of the genetic association between breast and ovarian cancer in some families. These findings highlight the importance of considering a woman's family history of ovarian cancer in the calculation of her risk of breast cancer. In this study, data on breast and ovarian cancer from the Cancer and Steroid Hormone Study, a large, population-based, case-control study conducted by the Centers for Disease Control, are used to calculate age-specific and cumulative risks of developing breast cancer for a woman with a first degree family history of ovarian cancer. These risks are calculated using maximum likelihood estimates from an autosomal dominant genetic model fit previously to the observed age-specific recurrence data of breast cancer among first degree relatives of the breast cancer cases and controls in these data as well as from genotype-specific estimates of lifetime ovarian cancer risk derived from this model. Under this model, the lifetime risk of developing breast cancer for a woman with one or two first degree relatives affected with ovarian cancer is estimated to be approximately 13% and 31%, respectively. A woman with one first degree relative affected with ovarian cancer and one first degree relative affected with breast cancer has an estimated risk of 40 percent of developing breast cancer by age 79 years if the relative with breast cancer was diagnosed in her thirties. This risk decreases with increasing age of onset of the relative affected with breast cancer.