The smallest subset of items from the 30-item Checklist for Autism Spectrum Disorder (CASD) that differentiated 607 referred children (3-17 years) with and without autism with 100% accuracy was identified. This 6-item subset (CASD-Short Form) was cross-validated on an independent sample of 397 referred children (1-18 years) with and without autism and on data from 1417 children in the CASD standardization sample and 1052 children in the CASD normative sample, resulting in 98.5, 97.6, and 99.8% diagnostic accuracy, respectively. Diagnostic agreement was high between the CASD-Short Form and the Autism Diagnostic Interview-Revised (96%), and the Child Autism Rating Scale (98%). Diagnostic accuracy for the CASD-SF was similar to accuracy for the 30-item CASD full form.