The quantification of differences in alpha electroencephalograph (EEG) activity between the eyes-closed and eyes-open resting conditions could be used as a measure of resting state arousal. The objective of this study was to evaluate the contribution of EEG alpha reactivity on opening the eyes, to the neurophysiology of children with attention-deficit hyperactivity disorder (ADHD). Thirty-eight children with ADHD were assessed using quantitative EEG (qEEG) analysis of absolute band power at rest, with eyes open and closed. Alpha reactivity index was calculated on opening the eyes, defined from the relationship between the absolute powers in the respective bands in the periods with the eyes open and closed. EEG data of 38 sex- and age-matched controls, with no neurological or psychiatric problems, were collected for comparison. There was a significant reduction in absolute alpha power at all electrodes for both ADHD and control groups with eyes open, indicating an increase in the arousal level. However, the alpha reactivity index was greater, corresponding to less reactivity, in the frontal regions of the children with ADHD (P < .01). Such a finding suggests alterations in arousal mechanisms in ADHD. This research suggests that alpha reactivity on opening the eyes, allied with other variables from the qEEG, may improve diagnostic accuracy in ADHD.