Introduction: The emergency department (ED) has long served as a safety net for the uninsured and those with limited access to routine healthcare. This study aimed to compare the characteristics and severity of ED visits in an Illinois academic medical center (AMC) and community hospital (CH) of a single health system before and after the implementation of the Affordable Care Act (ACA). Methods: This was a retrospective record review of 357,764 ED visits from January 1, 2011-December 31, 2016, of which 74% were at the AMC and 26% at the CH. We assessed the severity of ED visits by applying the previously validated Ballard algorithm, which classifies ED visits as non-emergent, intermediate, or emergent. Descriptive analyses were conducted to compare the characteristics of ED visits before and after the implementation of the ACA. We conducted multilevel logistic regression analysis to examine the odds of non-emergent compared to intermediate/ emergent ED visits by the ACA implementation status controlling for patient demographic characteristics, insurance status, and multiple visits per patient. Results: ED visits for patients with Medicaid or other governmental coverages increased in the post-ACA compared to pre-ACA period (Pre: 33.2 % vs Post: 38.3% at the AMC, and Pre: 29.7% vs Post: 35.1% at the CH). A statistically significant decrease in ED visits for uninsured patients was observed at the AMC and CH in the post-ACA period compared to the pre-ACA period (Pre: 12.1% vs Post: 6.4%, and Pre: 13.9% vs Post: 9.8%, respectively). Results from the regression analysis showed a significant decreased odds of non-emergent vs intermediate/emergent ED visits during the post-ACA period compared to the pre-ACA period at the AMC (odds ratio [OR] 0.68; confidence interval [CI], 0.66-0.70). However, an increased odds of non-emergent vs. intermediate/emergent ED visits was observed at the CH (OR 1.09; CI, 1.04-1.14). Conclusion: Similar to other Medicaid expansion states, ED utilization for uninsured patients decreased at both the AMC and the CH in the post-ACA period. While Medicaid visits for children < 18 years declined in the post-ACA period, it increased for ages 21 to 65 years of age. Contrary to our hypothesis, the severity of emergent ED visits increased in the post-ACA period but not at the CH.