Background: Supporting women to make well-informed decisions about breast cancer screening requires communicating that inconsequential disease may be detected, leading to overdiagnosis and overtreatment. Our trial investigated the effects of informing women about overdetection on their breast screening knowledge and participation over 2 years. Methods: This community-based, parallel-group, randomised controlled trial in New South Wales, Australia, recruited women aged 48–50, without personal or strong family history of breast cancer, who had not undergone mammography in the past 2 years. A computer program randomised 879 women to receive the intervention decision aid (evidence-based information on overdetection, breast cancer mortality reduction, and false positives; n=440) or a control decision aid (identical but without overdetection information; n=439). Participants and interviewers were blinded to group assignment. 838 women completed telephone interviews post-intervention (when we assessed our primary outcome, informed choice) and were contacted for follow-up at 6 months, 1 and 2 years. Outcomes were analysed in all women who completed relevant follow-ups. Main outcomes for this report are breast screening knowledge and participation. This trial is registered with ANZCTR.org.au, ACTRN12613001035718. Findings: We followed up 790 participants at 6 months, 756 at 1 year, and 712 at 2 years. Knowledge was superior in the intervention group at all time points. After 2 years, 123 (34%) of 358 women in the intervention group demonstrated adequate conceptual knowledge compared with 71 (20%) of 354 in the control group (odds ratio 2·04, 95% CI 1·46-2·85). Screening attendance over 2 years was similar between groups (200 [50%] vs 204 [51%]; odds ratio 0·97, 95% CI 0·73-1·29). Interpretation: A brief decision aid produced lasting improvement in women’s understanding of potential consequences of screening, including overdetection, without changing participation rates. These findings support the use of decision aids for breast cancer screening. Trial Registration: This trial is registered with ANZCTR.org.au, ACTRN12613001035718. Funding Statement: Australian National Health and Medical Research Council. Declaration of Interests: We declare no competing interests. Ethics Approval Statement: The University of Sydney Human Research Ethics Committee approved the trial (2012/1429), and the NSW Population & Health Services Research Ethics Committee additionally approved the linkage with BreastScreen NSW data (2018HRE0203).