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The effect of the Affordable Care Act on emergency department (ED) high utilizers has not yet been thoroughly studied. We sought to determine the impact of changes in insurance eligibility following the 2014 Medicaid expansion on ED utilization for ambulatory care sensitive conditions (ACSC) by high ED utilizers in an urban safety net hospital.High utilizers were defined as patients with ≥ 4 visits in the 6 months before their most recent visit in the study period (July–December before and after Maryland's Medicaid expansion in January 2014). A differences-in-differences approach using logistic regression was used to investigate if differences between high and low utilizer cohorts changed from before and after the expansion.During the study period, 726 (4.1%) out of 17,795 unique patients in 2013 and 380 (2.4%) of 16,458 during the same period in 2014 were high utilizers (p-value < 0.001). ACSC-associated visit predicted being a high utilizer in 2013 (OR 1.66 (95% CI [1.37, 2.01])) and 2014 (OR 1.65 (95% CI [1.27, 2.15])) but this was not different between years (OR ratio 0.99, 95% CI [0.72, 1.38], p-value 0.97).Although the proportion of high utilizers decreased significantly after Maryland's Medicaid expansion, ACSC-associated ED visits by high ED utilizers were unaffected.