A clinical study has been conducted on a selected group of 9 children and 12 adults with attention deficit disorder, evaluating the response to the therapy with magnesium pemoline by means of EEG power spectra. Pemoline was given orally in increasing doses ranging from 20 to 80 mg/day. EEG was recorded during baseline and at three fixed intervals during the treatment. Clinical response was assessed by objective and subjective ratings. In the adult sample, the drug produced two opposite patterns of EEG response which allowed us to discriminate by the 1st day of treatment responders who reported a paradoxical calming and non-responders who reported a worsening in anxiety. Inconsistent EEG power changes produced by the drug in children did not allow the discrimination of responders and non-responders. Children did not show paradoxical signs of sedation. EEG and clinical data suggest that adults and children show different drug response and that the paradoxical sedation is present only in adults.