Vineland Adaptive Behavior Scales were used to assess adolescents and young adults with Down syndrome or autism. Matched for verbal mental age (MA), the groups did not differ in adaptive behavior; however, older individuals with Down syndrome had more skills than did younger ones in all areas measured. Skills did not change for the autistic group. For persons with Down syndrome, adaptive skills kept pace with verbal and nonverbal MA, whereas those with autism were delayed in communication and socialization relative to their nonverbal MA.