BACKGROUND: While Medicaid expansion has improved access to primary care services, its impact on surgical specialty utilization remains unclear. To determine whether Medicaid expansion was associated with increased compared utilization rates of total hip arthroplasty (THA) and total knee arthroplasty (TKA) in Illinois (which expanded Medicaid) relative to Missouri (which did not expand Medicaid). METHODS: Using administrative data sources, we analyzed 374,877 total hospitalizations (236,333 in Illinois and 138,544 in Missouri) for THA/TKA from 2011-2016 (Illinois' Medicaid expansion date: 1/1/2014). RESULTS: The percentage of THA/TKA funded by Medicaid in Illinois was 2.4% in 2013 and 3.9% in 2016 (Missouri-2013: 2.7%; 2016: 2.6%). A difference-indifference analysis (adjusted for patient age and sex, county-level Area Deprivation Index, and number of orthopaedic *