We determined the validity of a parent-report questionnaire as a research tool for rating attention-deficit/hyperactivity disorder (ADHD) symptoms in children. Using Cohen's kappa and Pearson correlation, we examined the agreement between parent reports of ADHD symptoms (using the Swanson, Nolan and Pelham Questionnaire-IV; SNAP-IV) and clinical judgment (using a semi-structured parent interview). Also, we explored factors that may be associated with the level of agreement, using regression analyses. We found moderate levels of agreement for severity of overall ADHD (r=0.43) and for hyperactive-impulsive symptoms (r=0.54), but no significant agreement for inattentive symptoms. On individual symptom level (range kappa=-0.05-0.22) and for the presence/absence of ADHD (kappa=0.14), agreement was poor. Therefore, we conclude that parent-report questionnaires may be acceptable to rate the overall severity of ADHD symptoms in treatment effect studies, but not to detect the presence of ADHD in epidemiological studies.