In addition to expanding eligibility for Medicaid, the Affordable Care Act reformed the program's enrollment process, with the health insurance marketplaces playing a central role in the reforms. State-based marketplaces determine Medicaid eligibility, but federal regulations give states using the federal marketplace a choice either to allow the marketplace to make Medicaid eligibility determinations or to limit its role to assessing and referring applicants to the state Medicaid agency. This issue brief examines Medicaid enrollment data and finds that states that establish their own marketplaces realize higher Medicaid enrollment. In states that use the federal marketplace, Medicaid enrollment is higher when states have the marketplace determine eligibility. These findings underscore the importance of states' marketplace decisions regarding Medicaid enrollment.