AbstractBackgroundQuantifying the benefits and harms of breast cancer screening accurately is important for planning and evaluating screening programs and for enabling women to make informed decisions about participation. However, few cohort studies have attempted to estimate benefit and harm simultaneously.AimsWe aimed to quantify the impact of mammographic screening on breast cancer mortality and overdiagnosis using a cohort of women invited to attend Australia's national screening program, BreastScreen.MethodsIn a cohort of 41,330 women without prior breast cancer diagnosis, screening, or diagnostic procedures invited to attend BreastScreen Western Australia in 1994‐1995, we estimated the cumulative risk of breast cancer mortality and breast cancer incidence (invasive and ductal carcinoma in situ) from age 50 to 85 years for attenders and non‐attenders. Data were obtained by linking population‐based state and national health registries. Breast cancer mortality risks were estimated from a survival analysis that accounted for competing risk of death from other causes. Breast cancer risk for unscreened women was estimated by survival analysis, while accounting for competing causes of death. For screened women, breast cancer risk was the sum of risk of being diagnosed at first screen, estimated using logistic regression, and risk of diagnosis following a negative first screen estimated from a survival analysis.ResultsFor every 1,000 women 50 years old at first invitation to attend BreastScreen, there were 20 (95% CI 12‐30) fewer breast cancer deaths and 25 (95% CI 15‐35) more breast cancers diagnosed for women who attended than for non‐attendees by age 85. Of the breast cancers diagnosed in screened women, 21% (95% CI 13%‐27%) could be attributed to screening.DiscussionThe estimated ratio of benefit to harm was consistent with, but slightly less favourable to screening than most other estimates from cohort studies.ConclusionWomen who participate in organised screening for breast cancer in Australia have substantially lower breast cancer mortality, while some screen‐detected cancers may be overdiagnosed.