Autism spectrum disorders (ASD) are a class of conditions categorized by communication problems, ritualistic behaviors, and inappropriate social behaviors. While there is much evidence to support a genetic link for ASD, an identified genetic marker remains elusive. As such, practitioners place considerable emphasis on traditional measures of intelligence and adaptive behavior to aid in diagnosis. Despite the fact that these measures are commonplace, little research has been conducted to shed light on whether deficits in intellectual functioning affect the range of core symptoms for ASD. This study represents a first attempt to determine whether level of IQ has an effect on the expression of ASD symptoms in adults with intellectual disability (ID). Three hundred and six adults, 151 with both ASD and ID and 155 with ID alone, were evaluated with respect to the nature and extent of their ASD symptoms and intellectual functioning. Individuals with autism displayed a higher number of symptoms than those with pervasive developmental disorder-not otherwise specified (PDD-NOS) on all three domains of impairment (social, communication, repetitive behaviors). As expected, persons with ID alone evinced far fewer symptoms than both these groups. IQ level was found to be a moderator for expression of ASD symptoms for the entire sample but not for the autism group.