We tested the hypothesis that high activity levels in infancy would predict self-regulatory problems and later symptoms of attention deficit and hyperactivity disorder (ADHD) in a longitudinal study of British families (N = 321). Infants’ activity levels were assessed at 6 months, using 3 informants’ reports from the Infant Behavior Questionnaire (IBQ) and ActiGraphs during baseline, attention, and restraint tasks. At a mean of 33 months, the children were assessed on self-regulatory tasks; at a mean of 36 months, 3 informants reported symptoms of ADHD. At a mean of 7.0 years, the children were assessed on executive function tasks; 3 informants reported on the child's symptoms of ADHD; and diagnoses of disorder were obtained using the Preschool Age Psychiatric Assessment. Informants’ reports of high activity levels at 6 months predicted ADHD symptoms in early childhood and diagnoses of ADHD with clinical impairment at age 7. The IBQ activity scale was also associated with the children's later performance on self-regulation tasks in early and middle childhood. Activity level in infancy reflects normal variation and is not a sign of psychopathology; however, these findings suggest that further study of the correlates of high activity level in infancy may help identify those children most at risk for disorder.