Under the Affordable Care Act (ACA), Medicare coverage expanded in 2011 to fully cover annual preventive care visits. We assessed the impact of coverage expansion, using 2007-13 data from primary care patients of Medicare-eligible age at the Palo Alto Medical Foundation (204,388 patient-years), which serves people in four counties near San Francisco, California. We compared trends in preventive visits and recommended preventive services among Medicare fee-for-service and Medicare health maintenance organization (HMO) patients as well as non-Medicare patients ages 65-75 who were covered by private fee-for-service and private HMO plans. Among Medicare fee-for-service patients, the annual use of preventive visits rose from 1.4 percent before the implementation of the ACA to 27.5 percent afterward. This increase was significantly larger than was seen for patients in the other insurance groups. Nevertheless, rates of annual preventive care visit use among Medicare fee-for-service patients remained 10-20 percentage points lower than was the case for people with private coverage (43-44 percent) or those in a Medicare HMO (53 percent). ACA policy changes led to increased preventive service use by Medicare fee-for-service beneficiaries, which suggests that Medicare coverage expansion is an effective way to increase seniors' use of preventive services.