Previous ERP studies have provided mixed information about ADHD, especially in adults and when conscious attention to stimuli is not required. We used the auditory N1 to assess automatic attention in adults with and without ADHD. While participants watched a silent video, trains of 5 tones (400-ms onset-to-onset time) were presented with intertrain intervals (ITIs) of 1 or 5 s. The P1, N1, P2, and N2 were analyzed. Compared to controls, participants with ADHD had relatively little N1 attenuation after the 5-s ITI, which was driven by uniformly small N1s to all tones. However, after the 1-s ITI, the ADHD group had relatively large N2s to all 5 tones in the train. The reduced N1 in adults with ADHD indicated reduced automatic attention to salient sound stimuli, which may be due to reduced function of brain-stem arousal mechanisms. However, the increased N2 in these participants suggests they had developed certain compensatory mechanisms.