Americans who received public insurance under the Affordable Care Act use the emergency department (ED) more frequently than before they were insured. If newly enrolled patients cannot access primary care and instead rely on the ED, they may not enjoy the full benefits of health care services.The objective of the study is to characterize reasons for ED utilization among American adults by insurance status and usual source of care.Cross-sectional analysis of adult sample respondents to the 2013 National Health Interview Survey reporting 1 or more ED visits in the preceding 12 months.Among American ED users that reported no usual source of care and who reported relying on the ED, 27.7% (95% confidence interval [CI], 23.6%-32.2%) and 35.1% (95% CI, 28.0%-43.0%) noted at least 1 issue of access and none of acuity as a reason for their last ED visit, as compared to 17.7% (95% CI, 16.3%-19.2%) among those with a stable usual source of care.Although past research has shown that those who lack a stable usual source of care use the ED more often, this is the first population-level study to demonstrate their propensity for lack of access-based utilization. In the wake of the Affordable Care Act, EDs will need to evolve into outlets that service a wider range of health care needs rather than function in their current capacity, which is largely to address acute issues in isolation.