Quantitative variations in score profiles from the parent version of the PDD Behavior Inventory (PDDBI) were examined in young Autism and PDD-NOS groups defined by ADOS-G and ADI-R criteria, relative to a not spectrum (NS) group of similar age. Both the Autism and the PDD-NOS group profiles markedly differed from the NS group. The most sensitive measures of group differences were those domain and composite scores that assessed social communication competence, as well as the overall Autism Composite score. Sensitivity, specificity and positive and negative predictability measures were quite good for these measures. It was concluded that the PDDBI is useful in assisting in the differential diagnosis of autism spectrum disorder.