A longitudinal study of young attention-deficit hyperactivity disorder (ADHD) boys has found clear evidence for developmental abnormalities in event-related potential (ERP) waves that reflect cognitive processes associated with selective attentional tasks. Boys alternated attention to auditory or visual modalities in a train of stimuli, in an attempt to detect target stimuli in the attended modality. Results suggest that ADHD boys' attentional difficulties are due to insufficient facilitation of responses to the attended stimuli and not to an inability to block ignored stimuli. Abnormalities in ERPs reflecting cognitive processes associated with both interchannel selection mechanisms (processing negativity) and intrachannel selection mechanisms (P3b) were found. The degree of abnormality in the P3b responses to target stimuli in ADHD boys (lower than normal boys) was found to increase with age. It is suggested that the abnormally low P3b response to attended target stimuli found in ADHD boys may be due in part to insufficient LC noradrenergic activity normally triggered by attended task-relevant or novel stimuli.