Background: Previous cross-sectional data showed that children and adolescents with attention-deficit hyperactivity disorder (ADHD) are at increased risk of comorbid conduct, mood, and anxiety disorders as well as impairments in cognitive, social, family, and school functioning. However, longitudinal data were needed to confirm these initial impressions. Methods: Using DSM-III-R structured diagnostic interviews and raters blinded as to diagnosis, we reexamined psychiatric diagnoses at 1- and 4-year follow-ups in children with ADHD and controls. In addition, subjects were evaluated for cognitive, achievement, social, school, and family functioning. Results: Analyses of follow-up findings revealed significant differences between children with ADHD and controls in rates of behavioral, mood, and anxiety disorders, with these disorders increasing markedly from baseline to follow-up assessments. In addition, children with ADHD had significantly more impaired cognitive, family, school, and psychosocial functioning than did controls. Baseline diagnosis of conduct disorder predicted major depression and bipolar disorder at follow-up, and anxiety disorders at baseline predicted anxiety disorders at follow-up. Conclusion: These results confirm and extend previous retrospective results indicating that children with ADHD are at high risk of developing a wide range of impairments affecting multiple domains of psychopathology such as cognition, interpersonal, school, and family functioning. These findings provide further support for the value of considering psychiatric comorbidity in both clinical assessment and research protocols involving children with ADHD.