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Objective:To redefine overdiagnosis and reestimate the proportion of overdiagnosis of breast cancer caused by screening based on the Surveillance,Epidemiology,and End Results (SEER,1973-2015) Program data.Methods:The breast cancer diagnosed before 1977 was defined as the no-screening cohort since America had initiated breast cancer screening from 1977.The breast cancer diagnosed in 1999 was defined as the screening cohort due to no increases in both the proportion of early-stage breast cancer tmtil 1999 and the overall survival of early-stage breast cancer diagnosed over the three years since 1999.The magnitude of overdiagnosis was calculated as the difference in the proportions of early-stage breast cancer patients with long-time (15-year) survival to all breast cancer patients between two cohorts.Results:Over 23 years before and after widespread screening in America,the proportion of early-stage breast cancer patients increased from 52.1％ (16,891/32,443) to 72.7％ (16,021/22,025) (P＜0.001).The 15-year survival rate of early-stage breast cancer patients increased from 51.1％ to 61.5 ％ (P＜0.001),while the proportions of early-stage breast cancer patients with long-time survival to all breast cancer patients increased from 26.6％(52.1％×51.1％) to 44.7％ (72.7％×61.5％).Assuming no improvements in cancer screening technology and treatment technology,18.1％ (44.7％-26.6％) of breast cancer patients were overdiagnosed associated with screening.The age-specific overdiagnosis rates were 18.9％,24.7％,24.5％,20.5％,and 8.3％ for breast cancer patients aged 40-49,50-59,60-69,70-74,and ≥75 years old,respectively.Conclusions:Overdiagnosis caused by mammographic screening is probably overestimated in current screening practices.Further trials with more sophisticated designs and analyses are needed to validate our findings in the