1) Researchers administered the Autism Diagnostic Interview-Revised (ADI-R) to patients at a state psychiatric hospital to examine the validity and utility of the ADI-R for diagnosing autism spectrum disorder in adults with serious mental illness. 2) The ADI-R showed good agreement with clinical consensus diagnoses, with a kappa of 0.454. However, some items on repetitive behaviors and mannerisms may not differentiate autism from schizophrenia. 3) A high percentage of patients could not complete the full ADI-R due to lack of family informants or developmental history details, highlighting challenges in diagnosing autism in adult inpatient populations.