Early, accurate identification of ADHD would improve outcomes while avoiding unnecessary medication exposure for non-ADHD youths, but is challenging, especially in primary care. The aim of this paper is to test the Strengths and Difficulties Questionnaire (SDQ) using a nationally representative sample to develop scoring weights for clinical use. The British Child and Adolescent Mental Health Survey (N = 18,232 youths 5-15 years old) included semi-structured interview DSM-IV diagnoses and parent-rated SDQ scores. Areas under the curve for SDQ subscales were good (0.81) to excellent (0.96) across sex and age groups. Hyperactivity/inattention scale scores of 10+ increased odds of ADHD by 21.3x. For discriminating ADHD from other diagnoses, accuracy was fair (<0.70) to good (0.88); Hyperactivity/inattention scale scores of 10+ increased odds of ADHD by 4.47x. The SDQ is free, easy to score, and provides clinically meaningful changes in odds of ADHD that can guide clinical decision-making in an evidence-based medicine framework.