The present study examined diagnostic differentiation between school-aged children with autism spectrum disorders (ASD) and children with pragmatic language impairment (PLI). Standardized diagnostic instruments were used to investigate the relationship between severity of ‘autism triad’ impairments and group membership. The Autism Diagnostic Observation Schedule was administered to 19 children with PLI and 22 children with ASD. Parents completed the Social Communication Questionnaire. There was a significant difference between diagnostic groups in the level of the severity of behaviors represented by the Communication and Reciprocal Social Interaction sub-domains on both diagnostic measures. Currently displayed Repetitive and Restricted Behaviors and Interests were not found to be useful for differentiating between groups. The similarities found between groups have important implications for intervention.