Background: Adults with Autism Spectrum Disorders (ASD) represent a small, but challenging sub-group of patients within Ontario's mental health care system. However, few studies have documented the clinical characteristics of this population and examined how such individuals differ from other psychiatric patients, with or without intellectual disabilities (ID). Method: A secondary analysis of data from the 2003 Comprehensive Assessment Projects from three psychiatric hospitals in Ontario was conducted to describe patients with ASD and ID and to determine how their profile compared to other hospital users. Results: Twenty-three patients with ASD and ID were matched on gender and patient status (inpatient/outpatient) to individuals with and without ID. Individuals with ASD and ID were similar in terms of demographics to patients with and without ID. However, individuals with ASD and ID were younger, spent more days in hospital and were less likely to have a psychotic disorder diagnosis than both patients with and without ID. Inpatients with ASD and ID were recommended for a higher level of care than hospital service users without ID. Conclusions: Clearly, this small sub-group of individuals within the hospital population has high clinical needs that are not always well met. (Contains 4 tables.)