This report discusses the outcomes of a study that examined 216 comparably aged children and adolescents (ages 2-18) with fragile X or autism to determine whether longitudinal change in cognitive ability and adaptive behavior was similar in the two groups. Results found decreases in Intelligence Quotient scores in young children with fragile X as well as those with autism. Older children and adolescents with autism exhibited stable test-retest scores, while older children with fragile X continued to show decreases. Comparable declines in adaptive behavior were observed in both groups, at all ages, and across all adaptive behavior domains. (Contains 33 references.) (CR)