Autism spectrum disorders are often comorbid with other psychiatric symptoms and disorders. However, identifying psychiatric comorbidity in children with autism spectrum disorders is challenging. We explored how a questionnaire, the Child Behavior Check List, agreed with a Diagnostic and Statistical Manual of Mental Disorders–Fourth Edition (DSM-IV)-based semi-structured interview, the Kiddie-Schedule for Affective Disorders and Schizophrenia for School-Age Children (Kiddie-SADS). The sample comprised 55 children and adolescents (age 6 to 18 years) with autism spectrum disorders, including the main autism spectrum disorder subgroups and the broad range of cognitive and language functioning. High rate of psychopathology was found both through questionnaire and interview assessment. Using predefined Child Behavior Check List cutoffs, we found good agreement between the Child Behavior Check List and the Kiddie-SADS for identifying attention deficit/hyperactivity disorder, depressive disorders, and oppositional defiant disorder. However, overall the specificity of the Child Behavior Check List was low. The Child Behavior Check List was not useful for identifying anxiety disorders. The Child Behavior Check List may capture core symptoms of autism spectrum disorders as well as comorbid psychopathology, and clinicians should be aware that the Child Behavior Check List may be unspecific when used in children and adolescents with autism spectrum disorders.