Subspecialty physicians who have expertise in the diagnosis of autism spectrum disorder typically do not have the resources to administer comprehensive diagnostic observational assessments for patients suspected of ASD. The autism mental status exam (AMSE) is a free and brief eight-item observation tool that addresses this practice gap. The AMSE, designed by Child and Adolescent Psychiatrists, Developmental Behavioral Pediatricians and Pediatric Neurologists structures the observation and documentation of signs and symptoms of ASD and yields a score. Excellent sensitivity and specificity was demonstrated in a population of high-risk adults. This protocol now investigates the AMSE's test performance in a population of 45 young children age 18 months to 5 years with suspected ASD or social and communication concerns who are evaluated at an autism research center. Each subject received a developmental evaluation, including the AMSE, performed by a Child and Adolescent Psychiatrist, that was followed by independent standardized assessment using the Autism Diagnostic Observation Schedule and the Autism Diagnostic Interview-Revised. A Best Estimate Diagnosis protocol used DSM-5 criteria to ascertain a diagnosis of ASD or non-ASD. Receiver operating characteristic curve analysis was used to determine the AMSE cut point with the highest sensitivity and specificity. Findings indicate an optimized sensitivity of 94% and a specificity of 100% for this high prevalence group. Because of its high classification accuracy in this sample of children the AMSE holds promise as a tool that can support both diagnostic decision making and standardize point of care observational assessment of ASD in high risk children.