Changes in health, functional abilities, and behavior problems among 150 adults with Down syndrome and 240 adults with mental retardation due to other causes were examined with seven assessments over a 9-year period. Adults were primarily younger than 40, the age at which declines begin to be evident in individuals with Down syndrome. Adults with Down syndrome were advantaged in their functional abilities and lack of behavior problems, comparable in health, and exhibited comparable rates of change on these measures as adults with mental retardation due to other causes. Placement out of the parental home and parental death were predictors of change in health, functional abilities, and behavior problems.