Adults suffering from Attention Deficit Hyperactivity Disorder (ADHD) are known to have disturbed central dopaminergic transmission. With Single Photon Emission Computed Tomography (SPECT) we studied brain dopamine transporter and receptor activity in six boys with ADHD. Three months after initiation of treatment with methylphenidate we found a down-regulation of the post-synaptic dopamine receptor with a maximum of 20 % and a down-regulation of the dopamine transporter with a maximum of 74.7 % in the striatal system. This corresponded to a positive clinical response evaluated by neuropsychological questionnaires and tests. We suggest that dopamine transporter imaging by SPECT might be used to monitor psychostimulant treatment in children suffering from ADHD.