Mukama T, Kharazmi E, Xing X, et al. - In this nationwide cohort study involving nearly 5.1 million women born from 1932 onward and with at least one known first-degree relative (FDR) in Sweden, experts identified the risk-adapted starting age of breast cancer screening based on a woman’s detailed family history. A total of 118,953 received a diagnosis of primary invasive breast cancer. At the time of their diagnosis, 102,751 women did not have a family history of breast cancer in FDRs and second-degree relatives (SDRs). Risk-adapted starting age of breast cancer screening differed by the number of FDRs and SDRs with a breast cancer diagnosis and the age at diagnosis of the FDRs. When the youngest relative had a diagnosis following age 50 years, however, this risk level was achieved at age 36 years. Thus, this study recognizes potential risk-based starting ages for breast cancer screening on the basis of population-based registers. Moreover, these results may serve as high-quality evidence to strengthen prevailing screening guidelines for relatives of people with breast cancer.