Previous researchers have reported autistic features in children with fragile X syndrome. We compared 21 children with pervasive developmental disorders (autism group) to 15 with fragile X syndrome on the Childhood Autism Rating Scale and the Reiss Scales for Children's Dual Diagnosis. The 7 children (47%) with fragile X who scored above the Childhood Autism Rating Scale cut-off (fragile X-autism group) were more impaired than the remaining children (fragile X-no autism) on Childhood Autism Rating Scale subscales related to emotion, visual and listening responses, and communication. The autism group's Reiss scores were higher than fragile X-no autism group, but not fragile X-autism group. Although the Childhood Autism Rating Scale identified almost 50% of children with fragile X as having autism, qualitative differences may exist in specific autistic-like behaviors between children with autism and children with fragile X.