Background: Autism spectrum disorder (ASD) and attention deficit / hyperactivity disorder (ADHD) are frequently comorbid and, as both are defined as neurodevelopmental disorders in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, simultaneous diagnosis is possible. However, despite the frequency of this comorbid state, its endophenotypic features remain unclear. This study thus aimed to describe the behavioral and emotional problems in boys with comorbid ASD and ADHD using the Strengths and Difficulties Questionnaire (SDQ). Methods: In total, 102 boys (age, 6–12 years) diagnosed with one or both disorders were divided into three groups according to their clinical diagnosis: ASD + ADHD (N = 39), ASD (N = 37), and ADHD (N = 25). Symptoms and related behaviors were compared among the groups using parents’ ratings of the autism spectrum quotient, ADHD rating scale-IV, and SDQ. Results: In the ASD + ADHD group, the proportion of “clinical-range” cases was as high as 76.9% for the SDQ total difficulties score (TDS). The ASD + ADHD and ADHD groups had significantly higher TDS as well as behavioral problems and hyperactivity subscale scores than did the ASD group; however, the ASD + ADHD group did not have significantly different scores on any subscale compared with the other two groups. The ASD + ADHD and ASD groups also had significantly lower prosocial behavior scores than the ADHD group. Conclusions: When using the SDQ as a screening tool for neurodevelopmental disorders, a high TDS, conduct problems, hyperactivity, and low prosocial behavior can be considered characteristic of ASD and ADHD comorbidity in 6- to 12-year-old boys.
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