BACKGROUNDThere is a lack of evidence‐based recommendations for the age at which women with a family history of cancers other than breast cancer should start breast cancer screening.METHODSUsing Swedish family cancer data sets, the authors conducted a nationwide cohort study including 5,099,172 Swedish women born after 1931 (follow‐up, 1958‐2015). Accounting for calendar time, they calculated the relative risk of breast cancer for women with a family history of a discordant cancer in 1 first‐degree relative. Furthermore, the authors used 10‐year cumulative risk to determine the ages at which women with a family history of discordant cancer reached risk thresholds at which women in the general population were recommended to start breast cancer screening.RESULTSA family history of cancer at 15 sites was associated with an increased risk of breast cancer. Among women younger than 50 years, the highest risk of breast cancer was observed for those with a family history of ovarian cancer (standardized incidence ratio, 1.44; 95% confidence interval, 1.26‐1.64). In these women, the risk of breast cancer associated with a family history at other cancer sites ranged from 1.08‐fold for prostate cancer to 1.18‐fold for liver cancer. When breast cancer screening was recommended to be started at the age of 50 years for the general population, women with 1 first‐degree relative with ovarian cancer attained the threshold risk for screening at the age of 46 years. Women with a family history of other discordant cancers did not reach the risk thresholds for screening at younger ages.CONCLUSIONSMany cancers showed familial associations with breast cancer, but women with a family history of these cancers (except for ovarian cancer) did not reach risk thresholds for screening at younger ages.