In 2014 twenty-eight states and the District of Columbia had expanded Medicaid eligibility while federal and state-based Marketplaces in every state made subsidized private health insurance available to qualified individuals. As a result, about seventeen million previously uninsured Americans gained health insurance in 2014. Many policy makers had predicted that Medicaid expansion would lead to greatly increased use of hospital emergency departments (EDs). We examined the effect of insurance expansion on ED use in 478 hospitals in 36 states during the first year of expansion (2014). In difference-in-differences analyses, Medicaid expansion increased Medicaid-paid ED visits in those states by 27.1 percent, decreased uninsured visits by 31.4 percent, and decreased privately insured visits by 6.7 percent during the first year of expansion compared to nonexpansion states. Overall, however, total ED visits grew by less than 3 percent in 2014 compared to 2012–13, with no significant difference between expansion and nonexpansion states. Thus, the expansion of Medicaid coverage strongly affected payer mix but did not significantly affect overall ED use, even though more people gained insurance coverage in expansion states than in nonexpansion states. This suggests that expanding Medicaid did not significantly increase or decrease overall ED visit volume.