Anxiety is one of the most prevalent mental health challenges in autistic children, yet there is limited evidence on effective tools to measure treatment outcomes. Previous research with non-autistic children has found that the Child Anxiety Impact Scale, Parent Version achieved good diagnostic accuracy when measuring treatment outcomes and performed better than a commonly used symptom measure. However, this has not been evaluated for autistic children. The present study examined the psychometric properties of the Child Anxiety Impact Scale, Parent Version in autistic children and compared its utility against other anxiety symptom measures, to detect treatment outcomes as assessed by a gold-standard diagnostic interview, the Anxiety and Related Disorders Interview Schedule, Child Version, Parent Interview with the Autism Spectrum Addendum. Data were used from 212 children (aged 7-13 years) who participated in a randomised controlled trial. Receiver-operating characteristic curve analyses were conducted, and subsequent subgroup analyses were conducted using DeLong tests. Results demonstrated that the Child Anxiety Impact Scale, Parent Version had strong psychometric properties, with total scores significantly outperforming other measures in predicting post-treatment recovery from anxiety diagnoses. These findings have implications for future choices of treatment outcome measures in research and clinical practice.