Alterations in the X-linked gene MECP2 encoding the methyl-CpG-binding protein 2 have been linked to autism spectrum disorders (ASDs). Most recently, data suggest that overexpression of MECP2 may be related to ASD. To better characterize the relevance of MECP2 overexpression to ASD-related behaviors, we compared the core symptoms of ASD in MECP2 duplication syndrome to nonverbal mental age-matched boys with idiopathic ASD. Within the MECP2 duplication group, we further delineated aspects of the behavioral phenotype and also examined how duplication size and gene content corresponded to clinical severity. We compared ten males with MECP2 duplication syndrome (ages 3–10) with a chronological and mental age-matched sample of nine nonverbal males with idiopathic ASD. Our results indicate that boys with MECP2 duplication syndrome share the core behavioral features of ASD (e.g. social affect, restricted/repetitive behaviors). Direct comparisons of ASD profiles revealed that a majority of boys with MECP2 duplication syndrome are similar to idiopathic ASD; they have impairments in social affect (albeit to a lesser degree than idiopathic ASD) and similar severity in restricted/repetitive behaviors. Nonverbal mental age did not correlate with severity of social impairment or repetitive behaviors. Within the MECP2 duplication group, breakpoint size does not predict differences in clinical severity. In addition to social withdrawal and stereotyped behaviors, we also found that hyposensitivity to pain/temperature are part of the behavioral phenotype of MECP2 duplication syndrome. Our results illustrate that overexpression/increased dosage of MECP2 is related to core features of ASD. Autism Res 2012, ●●: ●●–●●. © 2012 International Society for Autism Research, Wiley Periodicals, Inc.