This study investigated EEG differences between two groups of children with attention-deficit/hyperactivity disorder combined type (ADHD), with or without comorbid oppositional defiant disorder (ODD), and normal control subjects. Each group consisted of 20 males. All subjects were between the ages of 8 and 12 years, and groups were matched on age. EEG was recorded during an eyes-closed resting condition from 21 monopolar derivations, which were clustered into nine regions for analysis. The EEGs were Fourier transformed to provide absolute and relative power estimates for the delta, theta, alpha and beta bands. Values were also calculated for the theta/alpha and theta/beta ratios. The ADHD groups had more absolute and relative theta than the control group. Regionally, the ADHD groups had less relative alpha and more relative delta in posterior regions, and less relative beta in the frontal regions, than the control group. These differences were also apparent in both ratio measures. Only two significant topographic differences were found between the ADHD groups, with both of these being less deviant from normality in the ADHD+ODD group than the ADHD group. These results indicate that EEG correlates of ADHD are not clouded by the presence of comorbid ODD, which suggests possible applications in clinical practice.