Rearrangements of chromosome 15q, including isodicentric 15 chromosomes and interstitial duplications and triplications, have been previously reported in association with autism spectrum disorders. We have identified two boys with exceptionally large der(15) chromosomes that are tricentric and contain four copies of the proximal long arm, including the Prader Willi/Angelman critical region, and leading to hexasomy of the involved segment. Biallelic inheritance of maternal alleles and methylation analysis indicate that the markers are maternally derived. Clinical assessment of the boys indicated severe cognitive impairment associated with marked delays in gross and fine motor skills. Social and language deficits were present in both, although the severity of the mental retardation precluded diagnosis of autism (both were considered to have pervasive developmental disorder-not otherwise specified). Neurologic manifestations included infantile spasms evolving into intractable early-onset myoclonic seizures, psychomotor regression, and profound diffuse hypotonia. These patients represent the most severe end of the spectrum of phenotypes associated with segmental aneuploidy for chromosome 15q11-q13.