Assessment of global functioning is an important consideration in treatment outcome research; yet, there is little guidance on its evidence-based assessment for children with autism spectrum disorders. This study investigated the utility and validity of clinician-rated global functioning using the Developmental Disability-Child Global Assessment Scale in a sample of higher functioning adolescents with autism spectrum disorders and comorbid anxiety disorders enrolled in a randomized controlled trial ("n" = 30). Pretreatment Developmental Disability-Child Global Assessment Scale scores correlated with severity of autism spectrum disorders core symptoms ("r" = -0.388, "p" = 0.034), pragmatic communication ("r" = 0.407, "p" = 0.032), and verbal ability ("r" = 0.449, "p" = 0.013) and did not correlate with severity of anxiety symptoms or with parent-reported adaptive behavior. Change in Developmental Disability-Child Global Assessment Scale scores during treatment was associated with autism spectrum disorders symptomatic improvement ("r" = 0.414, "p" = 0.040) and with improved general communication ("r" = 0.499, "p" = 0.013). Results support the importance of assessing global functioning in addition to symptom change and treatment response in clinical trials.