Individuals diagnosed with an intellectual disability (ID) share overlapping traits with those diagnosed with both ID and an Autism Spectrum Disorder (ASD). Therefore, the purpose of this study was to determine if caregivers' reports of symptoms of ASD are of value (i.e., when comparing them to clinical diagnoses of ASD) and to determine which symptoms of ASD best differentiate those with ASD from those with ID only. It was hypothesized that a subset of items would emerge using the Autism Spectrum Disorders-Diagnostic for Adults (ASD-DA) which would differentiate the two groups utilize in this study. One hundred eighty-six adults diagnosed with ID with and without diagnosis of ASD were assessed for core symptoms and other behavioral differences using the ASD-DA. Most of the individuals in the ID group (n=93) and the ID plus ASD group (n=93) were in the profound range of ID. The items which were most likely to distinguish individuals with ASD were in socialization domain. Furthermore, adults with ASD were more likely to be male and to be non-verbal. (Contains 2 tables.)