This study examined the effects of California's 2011 Medicaid expansion on inpatient utilization at safety net hospitals. It found that Medicaid admissions increased at safety net hospitals while uninsured admissions decreased. The proportion of admissions that were uninsured also declined significantly. Additionally, the number of preventable admissions decreased more for uninsured patients at safety net hospitals compared to non-safety net hospitals. However, the acuity level of Medicaid patients increased more at safety net hospitals. The changes suggest the Medicaid expansion improved payer mix for safety net hospitals but new Medicaid patients were sicker.