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Event-related auditory and visual potentials were recorded from 36 attention-deficit hyperactivity disorder (ADHD) and 35 normal 6-year-old subjects engaged in a two-choice discrimination task. When normal subjects attended to stimuli in a given modality, enhanced negative (N2) and positive (P3b) responses (as compared with responses to nonattended stimuli) were found for auditory and visual target stimuli. In contrast, when ADHD subjects attended, little or no enhanced negative responses were found in either modality, and enhanced positive P3b responses were found only in response to visual target stimuli. Auditory N1, N2, and P3b and visual N2 amplitudes to attended target stimuli were significantly reduced in ADHD subjects as compared with normal subjects. No between-group differences were found for responses to nonattended stimuli. Both amplitude and latency abnormalities indicate that ADHD boys suffer from deficient preferential processing of attended stimuli. P3b and N2 abnormalities found here suggest deficiencies in two independent cognitive processes thought to be crucial to what we perceive, learn, and remember.