Background: The aim of this study was to assess the validity of diagnosing attention deficit/hyperactivity disorder (ADHD) in high IQ children and to further characterize the clinical features associated with their ADHD. Methods: We operationalized giftedness/high IQ as having a full scale IQ greater than or equal to 120. We identified 92 children with a high IQ who did not have ADHD and 49 children with a high IQ that met diagnostic criteria for ADHD who had participated in the Massachusetts General Hospital Longitudinal Family Studies of ADHD. Results: Of our participants with ADHD and a high IQ, the majority (n = 35) met criteria for the Combined subtype. Relative to control participants, children with ADHD and high IQ had a higher prevalence rate of familial ADHD in first-degree relatives, repeated grades more often, had a poorer performance on the WISC-III Block Design, had more comorbid psychopathology, and had more functional impairments across a number of domains. Conclusions: Children with a high IQ and ADHD showed a pattern of familiality as well as cognitive, psychiatric and behavioral features consistent with the diagnosis of ADHD in children with average IQ. These data suggest that the diagnosis of ADHD is valid among high IQ children.