This study examined several questions pertaining to the Childhood Autism Rating Scale (CARS) in a sample of 274 preschool children (aged 2-6 years) clinically diagnosed as falling in one of five groups: Autistic Disorder, PDD-NOS, MR, Delayed, and Other. In addition to diagnosis and the CARS, all children were given standardized cognitive and adaptive behavior measures. Results indicated high concordance between the CARS and clinical diagnosis using DSM-IV (including excellent sensitivity and specificity). There was a moderate negative correlation of CARS scores and developmental level (both cognitive and adaptive), indicating significant shared variance. There were significant and sensible differences in mean CARS score for different diagnostic groups, including a substantial difference between the Autistic Disorder and PDD-NOS groups.