© 2016 American College of Emergency Physicians. Study objective The 2010 provision of the Patient Protection and Affordable Care Act (ACA) extended eligibility for health insurance for young adults aged 19 to 25 years. It is unclear, however, how expanded coverage changes health care behavior and promotes efficient use of emergency department (ED) services. Our objective was to use population-level emergency department data to characterize any changes in diagnoses seen in ED among young adults since the implementation of the ACA dependent coverage expansion. Methods We performed a difference-in-differences analysis of 2009 to 2011 ED visits from California, Florida, and New York, using all-capture administrative data to determine how the use of ED services changed for clinical categories after the ACA provision among young adults aged 19 to 25 years compared with slightly older adults unaffected by the provision, aged 26 to 31 years. Results We analyzed a total of 10,158,254 ED visits made by 4,734,409 patients. After the implementation of the 2010 ACA provision, young adults had a relative decrease of 0.5% ED visits per 1,000 people compared with the older group. For the majority of diagnostic categories, young adults' rates and risk of visit did not change relative to that of slightly older adults after the implementation of the ACA. However, although young adults' ED visits significantly increased for mental illnesses (2.6%) and diseases of the circulatory system (eg, nonspecific chest pain) (4.8%), visits decreased for pregnancy-related diagnoses and diseases of the skin (eg, cellulitis, abscess) compared with that of the older group (3.7% and 3.1%, respectively). Conclusion Our results indicate that increased coverage has kept young adults out of the ED for specific conditions that can be cared for through access to other channels. As EDs face capacity challenges, these results are encouraging and offer insight into what could be expected under further insurance expansions from health care reform.