The Autism-Spectrum Quotient is a 50-item questionnaire developed to assess autistic symptoms in adults, adolescents and children. Its original version and others in different countries are known to be effective tools in identifying individuals with autism spectrum disorder. This study examined whether the Hong Kong Chinese versions of the Autism-Spectrum Quotient-Child and Autism-Spectrum Quotient-Adolescent were effective in identifying autism spectrum disorder children and adolescents. On top of comparing them with their typically developing peers, this study also included a group of children/adolescents with attention deficit/hyperactivity disorder, a disorder with similar social difficulties as autism spectrum disorder. Results showed that both the Autism-Spectrum Quotient questionnaires were effective in differentiating the autism spectrum disorder group from the typically developing and attention-deficit/hyperactivity disorder groups, separately and jointly. On the contrary, they could not identify the attention-deficit/hyperactivity disorder group from the typically developing group so that they were not misclassifying attention-deficit/hyperactivity disorder as autism spectrum disorder. These findings supported that both the Autism-Spectrum Quotient-Child and Autism-Spectrum Quotient-Adolescent were not general measures of child and adolescent psychopathology, but could claim to be specific measures of autism spectrum disorder. Such capability would enormously enhance their utility in clinical practice for identifying autism spectrum disorder children/adolescents from their typically developing peers and from those with attention-deficit/hyperactivity disorder. This is because, the latter is a common neurodevelopmental disorder frequently presented to child psychiatric clinics alongside with autism spectrum disorder.