Cognitive ERPs and EEG spectral differences were compared in three groups of children: nonreferred controls, those with a dominant hyperactivity/impulsivity factor (ADHD-Im), and those with a dominant inattentive factor (ADHD-Ia). The results from the ERP analyses indicated that the P250, P350, and P500 components differed between the groups. The most marked differences were seen with respect to the amplitude of the P500 components. In addition, the topographic foci of the P500 components for the CON and ADHD-Im groups were symmetrical, but the ADHD-Ia group featured P250 and P350 components that were biased away from the right hemisphere. Nevertheless, the P500 was found to be an effective discriminator between the groups. The combined spectral and ERP results suggest that the attention disordered children have difficulty adjusting their level of physiological arousal, and are defective with respect to controlled (or effortful) processing.