Objectives To study the impact of eliminating cost sharing for screening mammography on mammography rates in a large Medicare Advantage (MA) health plan which in 2010 eliminated cost sharing in anticipation of the Affordable Care Act mandate. Study Setting Large MA health maintenance organization offering individual‐subscriber MA insurance and employer‐supplemented group MA insurance. Study Design We investigated the impact on breast cancer screening of a policy that eliminated a $20 copayment for screening mammography in 2010 among 53,188 women continuously enrolled from 2007 to 2012 in an individual‐subscriber MA plan, compared with 42,473 women with employer‐supplemented group MA insurance in the same health maintenance organization who had full screening coverage during this period. We used differences‐in‐differences analysis to study the impact of cost‐sharing elimination on mammography rates. Principal Findings Annual screening rates declined over time for both groups, with similar trends pre‐2010 and a slower decline after 2010 among women whose copayments were eliminated. Among women aged 65–74 years in the individual‐subscriber MA plan, 44.9 percent received screening in 2009 compared with 40.9 percent in 2012, while 49.5 percent of women in the employer‐supplemented MA plan received screening in 2009 compared with 44.1 percent in 2012, that is, a difference‐in‐difference effect of 1.4 percentage points less decline in screening among women experiencing the cost‐sharing elimination. Effects were concentrated among women without recent screening. There were no differences by neighborhood socioeconomic status or race/ethnicity. Conclusions Eliminating cost sharing for screening mammography was associated with modesty lower decline in screening rates among women with previously low screening adherence.