Objective: In 2003, Oregon's Medicaid expansion program, the Oregon Health Plan (OHP), implemented premiums and copayments and eliminated outpatient behavioral health services. We ascertained whether these changes, including $50 copayments for emergency department (ED) visits, affected ED use. Methods: This study used statewide administrative data on 414,009 adult OHP enrollees to compare ED utilization rates (adjusted for patient characteristics) in 3 time periods: (1) before the cutbacks, (2) after the cutbacks, and (3) after partial restoration of benefits. We examined overall ED visits and several subsets of ED visits: visits requiring hospital admission, injury-related, drug-related, alcohol-related, and other psychiatric visits. Because the policy changes affected only the expansion program (OHP Standard), we ascertained the impact of these changes compared with a control group of categorically eligible Medicaid enrollees (OHP Plus). Results: Compared with the control group, case-mix-adjusted ED utilization rates fell 18% among OHP Standard enrollees after the cutbacks. The rate of ED visits leading to hospitalization fell 24%. Injury-related visits and psychiatric visits excluding chemical dependency exhibited a similar pattern to overall ED visits. Drug-related ED visits increased 32% in the control group, perhaps reflecting the closure of drug treatment programs after the cutbacks reduced their revenue. Conclusion: The policy changes were followed by a substantial reduction in ED use. That ED visits requiring hospital admission fell to about the same extent as overall ED use suggests that OHP enrollees may have been discouraged from using EDs for emergencies as well as less-serious problems.