The age-associated incidence of significant decline in adaptive behavior and the temporal pattern of decline in specific functional skill domains were examined in 646 adults with mental retardation through 88 years of age. Cumulative incidence of significant decline for adults with Down syndrome increased from less than.04 at age 50 to.67 by age 72, whereas cumulative incidence of significant decline for adults with mental retardation without Down syndrome increased from less than.02 at age 50 to.52 at age 88. Among adults experiencing overall decline, four clusters of behaviors were identified based upon the sequence and magnitude of changes, suggesting a pattern of loss not unlike that noted in the population without mental retardation with dementia.