Six boys who all showed the combination of moderate-severe mental retardation, autistic behavior, and mild-moderate physical stigmatization are described. Muscular hypotonia, epilepsy, and kyphoscoliosis were associated features in several cases, as were extremes of short stature and low weight. A supernumerary chromosome was found in all six cases, and it appears that there may be a separate syndrome associated with partial trisomy of chromosome 15.