The present study examined the association between autism and sociodemographic factors, overall and in subgroups of children with autism with and without mental retardation (Autism/MR and Autism/No MR, respectively); the association was further examined in subanalyses by child's source of ascertainment to assess the presence of ascertainment bias. In the main analyses, one marker of higher social class (higher median family income) was significantly associated with autism overall. Both markers of higher social class (higher maternal education and higher median family income) were significantly associated with autism/no MR, but not associated with autism/MR. In the subanalyses, associations with social class varied by ascertainment source. Future studies should consider phenotypic subgroups of children with autism and must consider potential ascertainment bias.