This study investigated the childhood autism rating scale (CARS) as a tool for ASD diagnoses for 2-year-old (n = 376) and 4-year-old (n = 230) children referred for possible autism. The cut-off score to distinguish autistic disorder from PDD-NOS was 32 in the 2-year-old sample (consistent with Lord in J Child Psychol Psychiatry Allied Discipl, 36, 1365-1382, 1995), and 30 in the 4-year-old sample, with good sensitivity and specificity at both ages. The cut-off score to distinguish ASD from non-ASD at both ages was 25.5, with good sensitivity and specificity. Results confirm the utility of the CARS in distinguishing autistic disorder from PDD-NOS, and distinguishing ASD from other developmental disorders and typical development and suggest that an ASD cutoff around 25, which is in common clinical use, is valid.