The present study aimed to gain more insight in the social behavioral phenotype, and related autistic symptomatology, of children with an extra X chromosome in comparison to children with ASD. Participants included 60 children with an extra X chromosome (34 boys with Klinefelter syndrome and 26 girls with Trisomy X), 58 children with ASD and 106 controls, aged 9 to 18 years. We used the Autism Diagnostic Interview, Social Responsiveness Scale, Social Anxiety Scale and Social Skills Rating System. In the extra X group, levels of social dysfunction and autism symptoms were increased, being in between controls and ASD. In contrast to the ASD group, the extra X group showed increased social anxiety. The effects were similar for boys and girls with an extra X chromosome.