BACKGROUND Starting in September of 2010, the Patient Protection and Affordable Care Act required most health insurance policies to cover evidence-based preventive care with no cost-sharing (no copays, coinsurance, or deductibles). It is unknown, however, whether declines in out-of-pocket costs for preventive services are large enough to prompt increases in utilization, the ultimate goal of the policy. METHODS In this study, we use a nationally representative sample of ambulatory care visits to estimate the impact of the zero cost-sharing mandate on out-of-pocket expenditures on well-child and screening mammography visits. Estimates are made using 2-part interrupted time-series models, with well-woman visits serving as the control group because they were not covered under the zero cost-sharing mandate until after our study period. RESULTS Results indicate a substantial reduction in out-of-pocket costs attributable to the Affordable Care Act. Between January 2011 and September 2012, the zero cost-sharing mandate reduced per-visit out-of-pocket costs for well-child visits from $18.46 to $8.08 (56%) and out-of-pocket costs for screening mammography visits from $25.43 to $6.50 (74%). No reduction was apparent for well-woman visits. CONCLUSIONS The Affordable Care Act's zero cost-sharing mandate for preventive care has had a large impact on out-of-pocket expenditures for well-child and mammography visits. To increase preventive service use, research is needed to better understand barriers to obtaining preventive care that are not directly related to cost.