The life span has profoundly increased for individuals with Down syndrome (DS), leading to a large, understudied population of middle-aged and older adults with DS.1 Clinical studies show individuals with DS are at higher risk and have earlier onset of dementia.2 Down syndrome is defined by trisomy of chromosome 21, the site of the amyloid precursor protein gene. Overexpression of amyloid precursor protein–produced β-amyloid is hypothesized to increase risk for Alzheimer disease (AD) dementia in individuals with DS.3 Autopsy and neuroimaging studies indicate that by age 40 years, virtually all adults with DS exhibit AD neuropathology.2 However, individuals can live decades with AD neuropathology prior to developing clinical symptoms.4 Research examining prevalence of dementia in individuals with DS has been largely confined to clinic-based convenience samples.3 Population-based epidemiological work is needed to clarify the extent of dementia in DS and illustrate the public health outcomes for the DS population, their families, and health service systems. We describe prevalence and incidence of dementia and AD in DS in a full Medicaid population of adults with DS in Wisconsin from 2008 through 2018.