Familial breast cancer in a population-based series.

Breast cancer risks to sisters of breast cancer patients were examined in a population-based series of patients diagnosed in Los Angeles County between 1971 and 1975. Sisters of bilateral patients diagnosed at age 50 years or younger had substantially increased risk (relative risk (RR) = 5.5), and risk was even higher for sisters of bilateral patients diagnosed at age 40 years or younger (RR = 10.5). Half of the breast cancers in sisters of bilateral cases occurred in the family of a single bilateral patient whose disease was diagnosed at age 39 years. Sisters of unilateral patients diagnosed at age 50 years or younger did not have significantly increased risk, but sisters of unilateral patients diagnosed at age 40 years or younger appeared to have increased risk (RR = 2.4). Risk to sisters of bilateral patients was slightly higher if the patient's contralateral diagnoses were less than three years apart than if they were three years apart or greater (RR = 6.3 vs. 3.9), but this difference was not statistically significant.