People with intellectual and developmental disabilities (IDD) are frequent users of health services. We examined how their service utilization of emergency department (ED), inpatient hospitalization, and primary care physicians changed as they transitioned from fee-for-service to Medicaid managed care (MMC). Our results showed that MMC reduced the utilization of all of these services. A substantial decrease in ED visits was associated with the reduction in visits due to mental/behavioral health conditions and conditions that could be nonemergent and manageable with the community-based health services. These findings suggest that health service utilization of people with IDD is related not only to their health needs, but also to the delivery model that provides their health services.