This report describes differences in motoric and instrumental activity of daily living skills (MADLs and IADLs) between 1,442 people with autism and 24,048 people with mental retardation, using data from an adaptive behavior measure. Comparisons were made using groups defined by age (5-12, 13-21, and 21-35 years) and intellectual level. Diagnoses of record were confirmed through group analyses of rates of problem behaviors consistent with autism and comparison to an independent data base. Findings suggest that at ages 5-12 the skills of children with autism are more developed than those of children with mental retardation matched by age and intellectual level. However, in the older groups these differences diminish, and with increasing age (21-35 years) more developed instrumental skills are observed for people with mental retardation.