The reliability of a new scale to assess Autistic Disorder, Pervasive Developmental Disorder, Not Otherwise Specified (PDD-NOS), and Asperger's Disorder in children was examined. Parents or other caregivers rated symptoms of 207 children between 2 and 16 years of age. The scale, which had 40 items in the final version, correlated highly with DSM-IV-TR and ICD-10 criteria and proved to have good inter-rater reliability and, excellent test-retest, and internal consistency reliability. These data are considered to have clinical utility given the need to establish data-based distinctions between these three subtypes of ASD. Furthermore, the measure is part of a more extensive battery measuring comorbid psychopathology and challenging behaviors. Implications for future research of this comprehensive assessment battery are discussed. (Contains 4 tables.)