Background: Having a family history of breast cancer, particularly if it involves early-onset disease, is a risk factor for breast cancer, but little is known about specific causes of this association. Consequently, we studied mothers, sisters, and aunts of an age-stratified sample of 1567 unselected case patients diagnosed with breast cancer before age 60 years, recruited to a population-based, case-control-family study, in which case patients, control subjects, and their relatives were administered the same questionnaire. Methods: Extensive BRCA1 and BRCA2 mutation testing was carried out for 788 case patients diagnosed before age 40 years, including manual sequencing of DNA from 72 patients with two or more affected relatives. Standardized morbidity ratios, age-specific cumulative risks, and hazard ratios were calculated for groupings of relatives. Results: Cumulative risks of breast cancer to age 50 years in the sisters, mothers, and aunts of the case patients, respectively, were 6, 3, and 2 times the population risk if the case patient was younger than age 40 years at diagnosis but were considerably lower if the case patient was older at diagnosis. When relatives of the case patients with a BRCA1 or BRCA2 mutation were excluded, these risks fell by, at most, 20%. Sisters and aunts, but not mothers, who had an additional first-degree relative with breast cancer were at increased risk, and the risk was greater when that relative was younger at diagnosis. Hazard ratios were 10.7 (95% confidence interval [CI] = 4.2 to 26.8) for sisters and 4.2 (95% CI = 2.2 to 8.1) for aunts, if the relative was aged 40 years at diagnosis. Fewer than one-third of the excess of breast cancers in relatives of case patients diagnosed before age 40 years that are attributed to familial factors are BRCA1- or BRCA2-related. Conclusion: Mutations in genes other than BRCA1 and BRCA2 may be associated with a high risk of breast cancer, especially in young women.