Gaps in knowledge and systematic tracking of the prevalence of intellectual and developmental disabilities (IDD) and characteristics that may affect the health of this disability group limits our ability to address the health disparities they experience in comparison to people without disability. The purpose of this study is to begin to fill one relevant critical gap in knowledge: understanding the demographics and health outcomes of adults with IDD who receive services under Medicare Fee-for-Service (FFS), many of who are also eligible for Medicaid. Using 2016 Medicare administrative claims, we examined the prevalence and characteristics of five diagnosis groups of IDD, in those under 65 and those 65 and over, as well as their health outcomes. We found that the IDD Medicare FFS group had high prevalence rates for chronic physical and mental health conditions, overuse of emergency departments, and high rate of 30-day readmission. These findings highlight the need for evidence-based health care coordination, improved and increased public health interventions, and continued surveillance.