We use four states that were early adopters of Medicaid expansion to study how this expansion affects enrollment and access to physicians for Medicaid enrollees. We use the universe of Medicaid enrollment and claims data to construct state-month-level measures of enrollment, enrollee composition, and access to physicians. Using a differences-in-differences framework, we find that Medicaid expansion leads to a 13 percent increase in overall enrollment, a 27 percent increase in enrollment among adults ages 23 to 65, and a 7 percent increase in the number of Medicaid patients seen by physicians. We find no statistically significant increase in the number of Medicaid patients seen among obstetricians/gynecologists and pediatricians, who are less likely to be affected by the expansion. We find that Medicaid expansion increases physician participation on the intensive margin but not on the extensive margin.