Objective: The social disability associated with ADHD often makes diagnostic and treatment decision making challenging. This protocol investigates the test performance of the Autism Mental Status Exam (AMSE) in detecting autism spectrum disorder (ASD) in a sample of 45 children with ADHD and ASD symptomatology. The AMSE is a brief ASD diagnostic assessment administered in the context of a clinical exam. Method: All participants received a developmental evaluation, including the AMSE, followed by independent gold standard diagnostic assessments including the Autism Diagnostic Observation Schedule (ADOS) and the Autism Diagnostic Interview–Revised (ADI-R). Results: Receiver operating characteristics (ROC) curve analysis indicated strong sensitivity and specificity in this population. Optimal cutoff scores are provided. Conclusion: The AMSE holds promise as a brief ASD assessment tool for children with ADHD and ASD symptomatology and as a guide for treatment and referral decisions at the point of care.