Pallister–Killian syndrome is a sporadic disorder caused by the presence of mosaic tetrasomy of the short arms of chromosome 12. Case reports of children with Pallister–Killian syndrome have described a range of developmental and behavioral outcomes, but no systematic studies of these outcomes exist. The objective of this study was to describe developmental and behavioral characteristics of individuals with Pallister–Killian syndrome participating in a national meeting of families and their affected children. Sixteen individuals with Pallister–Killian syndrome, ages 16 months to 19 years, were studied using questionnaires and direct interview. Among the 16 patients enrolled in the study, 3 probands were between 16 and 19 months, and had severe developmental delay. Among the rest of the 13 probands older than 24 months, 11 children had a developmental level of less than 8 months age equivalent. They were non-ambulatory, non-verbal, and passive, requiring extensive assistance in daily living. There were two higher functioning children who were ambulatory, and verbal. One of these children met criteria for Autism on the Autism Diagnostic Interview-Revised. Thus, although most individuals with Pallister–Killian syndrome studied showed profound intellectual disability and sensory impairments, individuals with Pallister–Killian syndrome can have mild to moderate intellectual disability. Therefore, in individuals with physical examination findings of Pallister–Killian syndrome, formal diagnostic testing should be considered, even in individuals with mild to moderate intellectual disability. Further studies will be needed to determine if these higher functioning children with Pallister–Killian syndrome are at increased risk for autism. © 2012 Wiley Periodicals, Inc.