Predictive validity is a fundamental consideration in evaluating the DSM-IV diagnostic criteria for attention deficit hyperactivity disorder (ADHD), particularly for younger children. The authors conducted four annual assessments of ADHD and functional impairment using multiple informants in 255 probands and matched comparison children who were 4–6 years old in wave 1. Nearly all children who met full criteria for ADHD in wave 1 met full criteria for ADHD over the next 3 years and continued to display marked functional impairment relative to comparison children, even when intelligence, co-occurring psychopathology, and demographic characteristics were controlled. These findings support the validity of the DSM-IV diagnosis of ADHD in younger children by demonstrating that the symptoms and associated impairment are likely to persist well into elementary school.