Background: The purpose of this study was to report on the psychometric measures and discriminatory function of a new diagnostic test for autism spectrum disorders, the Clinical Autism Diagnostic Scale (CADS). Methods: The CADS was used to test 216 children in the study, including 86 with low-functioning autism specturm disorders (ASD), 16 children with highfunctioning ASD, 16 with pervasive developmental disorder, not otherwise specified, 7 with Asperger syndrome, 65 with typical development, 11 children with language impairments and 15 with intellectual disabilities. Ages ranged from 38-73 months. Behaviors for the groups were compared across seven domains. Results: The results indicated the instrument was reliable, valid, and successfully differentiated the different groups of children with and without autism. All ASD groups were found to display difficulties in the domains of sensory behaviors and stereotyped behaviors. The play and social domains were found to measure similar underlying concepts of behaviors, while the receptive language and expressive language domains were also found to measure similar underlying-language concepts. The group of children diagnosed as having low-functioning autism performed less well on all tested domains in the instrument than did the other three groups of children with ASD, and these other three groups each also presented unique patterns of behaviors and differed on individual domains. Conclusions: CADS is a reliable and valid test. It successfully differentiates the abilities of children with ASD at different levels of functioning.