The Medical Services Initiative program--a safety net-based system of care--in Orange County included assignment of uninsured, low-income residents to a patient-centered medical home. The medical home provided case management, a team-based approach for treating disease, and increased access to primary and specialty care among other elements of a patient-centered medical home. Providers were paid an enhanced fee and pay-for-performance incentives to ensure delivery of comprehensive treatment. Medical Services Initiative enrollees who were assigned to a medical home for longer time periods were less likely to have any emergency room (ER) visits or multiple ER visits. Switching medical homes three or more times was associated with enrollees being more likely to have any ER visits or multiple ER visits. The findings provide evidence that successful implementation of the patient-centered medical home model in a county-based safety net system is possible and can reduce unnecessary ER use.