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Symptoms of autism spectrum disorder and attention-deficit/hyperactivity disorder often co-occur, challenging timely, and accurate diagnosis. We assessed the performance of three parent-report measures in discriminating autism spectrum disorder from attention-deficit/hyperactivity disorder without autism spectrum disorder (ADHDw/oASD) in school-age verbally fluent children. We examined the Autism Symptom Interview – School-Age and two widely used parent questionnaires: Social Responsiveness Scale – 2nd Edition and Social Communication Questionnaire – Lifetime. Receiver operating characteristic curves assessed each instrument’s performance against the best-estimate clinician Diagnostic and Statistical Manual of Mental Disorders, 5th Edition diagnosis of autism spectrum disorder or of ADHDw/oASD ( n = 74, n = 102, respectively; 6–11 years). These yielded moderate accuracies: area under the curve = 0.85, 0.79, and 0.78 for Social Communication Questionnaire – Lifetime, Autism Symptom Interview, and Social Responsiveness Scale – 2nd Edition, respectively. Area under the curve pairwise comparisons reached our statistical significance ( p w/oASD. Lay abstract We tested the ability of a short, recently developed parent interview and two widely used parent-report questionnaires to discriminate school-age verbal children with autism spectrum disorder from those with attention-deficit/hyperactivity disorder without autism spectrum disorder (ADHDw/oASD). These measures included the Autism Symptom Interview – School-Age, the Social Responsiveness Scale – 2nd Edition, and the Social Communication Questionnaire – Lifetime. The classification accuracy of all three parent screeners fell in the moderate range. Accuracy varied by instrument, and the Social Communication Questionnaire – Lifetime questionniare showed the highest accuracy. Children with autism spectrum disorder who were incorrectly classified by all parent screeners did not differ from those correctly classified in regard to demographics, intellectual abilities, nor in any specific clinical area beyond general parent concerns. These findings showed that there are valid screening options for assessing school-age verbal children with autism spectrum disorder versus ADHDw/oASD. They also underscore the need to assess multiple sources of information for increased accuracy.