The Autism Spectrum Disorders-Diagnostic for Children (ASD-DC) is a 40-item Likert format scale designed to serve in the diagnosis of children and adolescents from 2 to 16 years of age. The reliability and factor structure of the scale have been established in previous research. Studies 1 and 2 were designed to evaluate the validity of the measure by establishing cut-off scores for 161 participants falling into the categories of typical development, atypical development/psychopathology, and ASD (i.e., autism, PDD-NOS, and Asperger's syndrome), as assessed by a licensed psychologist using ICD-10 and DSM-IV-TR criteria and in conjunction with standardized measures of autism (ADI-R, CARS, or CHAT) and Asperger's syndrome (CAST, GADS, or KADI). In study 3, the accuracy of ASD scores was compared with ICD-10 and DSM-IV-TR criteria for autism, PDD-NOS, and Asperger's syndrome in 219 children. The ASD-DC, which can be administered in 10-15 min proved to be a relatively accurate and valid diagnostic instrument when compared to the diagnostic methods described above. The implications of these data for further development of this scale are discussed. (Contains 5 tables.)