Attention-deficit/hyperactivity disorder (ADHD) is one of the most common childhood-onset psychiatric disorders. It is distinguished by symptoms of inattention, hyperactivity, and impulsivity. ADHD may be accompanied by learning disabilities, depression, anxiety, conduct disorder, and oppositional defiant disorder. The etiology of ADHD is unknown, and the disorder may have several different causes. Investigators have studied, for example, the relation of ADHD to elevated lead levels, abnormal thyroid function, morphologic brain differences, and EEG patterns. With current public awareness of ADHD, pediatricians and health care providers are reporting increases in referral rates of children with suspected ADHD. Numerous rating scales and medical tests for evaluation and diagnosis of ADHD are available, with mixed expert opinion on their usefulness. The Agency for Health Care Policy and Research sponsored development of this technical review to summarize current scientific evidence from the literature on the prevalence of ADHD and on the value of various evaluation methods. The following questions provided a framework for the analysis: 1. What percentage of the U.S. general population aged 6 to 12 years has ADHD? Of those with ADHD, what percentage has one or more of the following comorbidities: learning disabilities, depression, anxiety, conduct disorder, and oppositional defiant disorder? 2. What percentage of children aged 6 to 12 years presenting at pediatricians' or family physicians' offices in the United States meets diagnostic criteria for ADHD? Of those with ADHD, what percentage has one or more of the following comorbidities: learning disabilities, depression, anxiety, conduct disorder, and oppositional defiant disorder? 3. What is the accuracy (Le., sensitivity, specificity, positive predictive value) and reliability (i.e., inter/intra-rater agreement) of behavioral rating screening tests for ADHD compared with a reference standard? 4. What is the prevalence of abnormal findings on selected medical screening tests commonly recommended as standard components of an evaluation of a child with suspected ADHD?