Despite considerable interest in Rett syndrome, there have been few studies of associated behavioral and emotional problems. In the present study, 143 girls with Rett syndrome were compared on the Developmental Behavior Checklist with 85 girls with severe to profound mental retardation of mixed etiologies. After controlling for the effects of physical disabilities, we found that the girls with Rett syndrome presented more "autistic-relating" and fewer antisocial behaviors. A subsample of children with autism was also compared to the girls with Rett syndrome on autistic-relating behaviors, revealing that the Rett syndrome group did not present with classic autistic behavioral features The implications of these results for the identification of a Rett syndrome behavioral phenotype are discussed.