Concerns have been raised regarding the validity of the diagnosis of attention-deficit hyperactivity disorder in adults. The purpose of this report is to evaluate critically whether this diagnosis in adults meets acceptable standards for diagnostic validity. A systematic search was conducted of the psychiatric and psychological literature for empirical studies dealing with adult attention-deficit hyperactivity disorder with childhood onset. These studies were examined for evidence of descriptive, predictive, and concurrent validity. The literature shows that this disorder can be reliably diagnosed in adults and that the diagnosis confers considerable power to forecast complications and treatment response. In addition, evidence is mounting for genetic transmission, specific treatment responses, and abnormalities in brain structure and function in affected individuals. Evidence from the literature is increasingly pointing to the validity of adult attention-deficit hyperactivity disorder.