Abstract. Objective: To determine whether the advent of a mandatory Medicaid managed care (MMC) plan had any effect on emergency department (ED) utilization by adult Medicaid patients at an urban teaching hospital. Methods: This was a retrospective cohort study using four years of ED records encompassing the year prior to initiation of MMC (1994-95), the enrollment year (1995-96), and two years after the program had matured (1996-98). Results: Total ED census declined slightly, then returned to 1995 levels. Emergency department use by MMC patients declined steadily, with the 1998 figure of 5,888 representing a 40% decline over the pre-MMC volume of 9,849. Visits by MMC patients with acute illness or injury declined by 29%; MMC low-acuity visits decreased by 43%. Medicaid managed care low-acuitye after-hours/weekend visits declined by 19%, then leveled off. The MMC enrollment was stable throughout the study period. Conclusions: Mandatory managed care can be associated with considerable diminution in ED use by Medicaid patients. This decline is most pronounced in low-acuity triage categories, and least evident after hours and on weekends.