The expansion of health insurance to millions of Americans through the Affordable Care Act has given rise to concerns about increased use of emergency department (ED) and hospital services by previously uninsured populations. Prior research has demonstrated that continuity with a regular source of primary care is associated with lower use of these services and with greater patient satisfaction. We assessed the impact of a policy to increase patients' adherence to an individual primary care provider or clinic on subsequent use of ED and hospital services in a California coverage program for previously uninsured adults called the Health Care Coverage Initiative. We found that the policy was associated with a 42 percent greater probability of adhering to primary care providers. Furthermore, patients who were always adherent had a higher probability of having no ED visits (change in probability: 2.1 percent) and no hospitalizations (change in probability: 1.7 percent), compared to those who were never adherent. Adherence to a primary care provider can reduce the use of costly care because it allows patients' care needs to be managed within the less costly primary care setting.