According to the major classification systems it is not possible to diagnose a comorbid autistic disorder in persons with Rett syndrome. However, this is a controversial issue, and given the level of functioning of persons with Rett syndrome, the autistic disorder is expected to be present in a comparable proportion as in people with the same level of functioning. To investigate, parents of 52 females with classical and atypical Rett syndrome (2.4—49.3 years) completed the Developmental Behavior Checklist (DBC), the Diagnostic Interview for Social and Communication Disorders (DISCO) and the Dutch Vineland Screener 0—6 (VS 0—6). All participants had a severe to profound intellectual disability (ID) according to the VS 0—6. Behavior indicated an autistic disorder in 42 (DBC) to 58 percent (DISCO) of the Rett cases. Autistic behavior had decreased in 19 percent such that they no longer met the criteria for autistic disorder. Some participants were suspected of having a comorbid autistic disorder, though not more often than can be expected at their level of functioning. Clinicians should be aware of the possibility of a comorbid autistic disorder as much as they should be in other people with this level of functioning.