Anxiety disorders are present in up to 80% of youth diagnosed with autism spectrum disorders, but the nature of anxiety in ASD is not well understood. The symptoms of anxiety may be difficult to separate from symptoms of autism. This is especially true of atypical fears and worries – which could be part of anxiety or autism or possibly both. An instrument designed and validated to differentiate and comprehensively assess symptoms of anxiety in ASD is needed. The ADIS is currently the ‘gold standard’ in assessing anxiety in children, but requires adaptation to differentiate and assess the full range of anxiety symptoms apparent in ASD. This study will provide resources to validate the Autism Spectrum Addendum (ASA) to the Anxiety Disorders Interview Schedule (ADIS). Clarifying the differential diagnosis of anxiety and ASD symptoms and assessment of atypical fears via a validated instrument has important implications for research and clinical practice – including proper treatment and intervention strategies for people with ASD and anxiety problems.