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This study aims to review the nosology and epidemiology of attention-deficit/hyperactivity disorder (ADHD) in children aged 2 through 5 years. Studies, primarily in community or pediatric clinic settings, were reviewed. In studies using DSM diagnostic criteria, the prevalence of ADHD in preschool children ranges from 2.0% to 7.9%, with hyperactive-impulsive type and combined type significantly more common than pure inattentive type ADHD. Boys and older preschoolers (4- and 5-year-olds vs 2- and 3-year-olds) are more likely to meet criteria for ADHD. Preschoolers with ADHD are significantly impaired in their relationships with adults and other children, in their functioning at home and outside the home, and in cognitive and academic performance. Preschoolers with ADHD are significantly more likely to meet criteria for other psychiatric disorders and those who do meet criteria for other psychiatric disorders are more severely impaired than preschoolers with ADHD alone. Despite the severity of impairment, only about a quarter of preschoolers with ADHD are referred for mental health evaluation or treatment. Preschool ADHD predicts future ADHD and persistent impairment. Preschoolers with ADHD look like older children with ADHD with similar symptom presentations, associated features, and prognosis. These findings suggest that DSM-IV-TR ADHD is a reliable and valid diagnosis for children aged 2 through 5 years.