We examined the prevalence and clinical characteristics of oppositional defiant disorder (ODD) in a sample of clinically referred adults with attention deficit hyperactivity disorder (ADHD). Subjects were consecutively referred adults with a DSM-III R/IV diagnosis of ADHD with or without ODD. Nearly half of subjects (43%) had a history of ODD. Subjects with a childhood history of ODD had increased risk for bipolar disorder, multiple anxiety disorders, and substance use disorders relative to the ADHD subjects without ODD. We concluded, as in children with ODD, adults with a childhood history of ODD have high rates of psychiatric comorbidity and more impaired psychosocial functioning than those without this condition. A better understanding of the course, phenomenology, and clinical significance of ODD in adults is needed to better understand therapeutic approaches for this disorder.