The Affordable Care Act (ACA) mandated coverage of common preventive services with zero patient cost sharing. However, patients may still experience high same-day costs when receiving these "zero-dollar" preventive services. Our analysis of on- and off-exchange individual-market health plans during 2016-18 revealed that 21-61 percent of enrollees experienced same-day cost exposure greater than $0 when accessing ACA-mandated free preventive services.