At least three research groups have reported that autism is diagnosed in up to 20% of children with velocardiofacial syndrome (VCFS). However the degree of phenotypic overlap between VCFS-affected children with autism and those with idiopathic autism has not been established. The purpose of this study was to define and differentiate the behavioral phenotype of autism in samples of children with either (VCFS) or idiopathic autism. Five groups of children ages 5–15 were included in the between-group design. Parent report of autism behaviors (based on the Autism Diagnostic Interview-Revised, ADI-R) were compared between children with VCFS, children with VCFS and autism (VCFS + autism), siblings of the children with VCFS, a community control group, and a group of children with idiopathic autism. Autism diagnoses were based according to the ADI-R. Parental responses to the ADI-R indicated that relative to children with VCFS-only, children with idiopathic autism and children with VCFS + autism exhibited less make believe play and more rituals, motor stereotypies and repetitive use of objects. However several other core autism behaviors, including difficulties sharing attention, deficits in gestural communication and initiating conversation, and presence of circumscribed interests, appear to be phenotypic VCFS behaviors, characterizing children with VCFS regardless of an autism diagnosis. Accordingly, the autism phenotype in VCFS differs to some extent from that of idiopathic autism. Several features of idiopathic autism are spared in VCFS, and other features appear to be a function of the VCFS phenotype independent of autism. These findings carry implications for clinicians who diagnose and treat VCFS or autism, and for researchers who study genotype–phenotype associations in autism. © 2007 Wiley-Liss, Inc.