Background: Autism is one of the most intriguing and highly studied conditions in the field of mental health. However, most research has been carried out with young children to the exclusion of adults, particularly adults with intellectual disability (ID). The present study represents a first attempt to describe symptom patterns of autism for adults with severe ID. Method: Fifty-seven adults with ID who met DSM-IV-TR and ICD-10 diagnostic criteria for autism were compared to 57 matched controls with ID only (no Axis I diagnosis). Results: Marked differences were noted in a range of symptoms, particularly with respect to impairments in social interaction and restricted or repetitive behaviours and interests. These items were able to predict the presence or absence of autism at a high rate of diagnostic accuracy. Conclusions: Based on these findings, it appears that adults with ID and autism evince a distinct pattern of impairment when compared to controls with ID alone. (Contains 2 tables and 1 figure.)