The purpose of this study was to estimate striatal dopamine (D2) receptor availability in non-drug treated children with attention-deficit-hyperactivity disorder (ADHD) before and after methylphenidate therapy, and to examine correlations between severity of symptoms and response rates to stimulant medication with levels of striatal D2 receptor binding. Nine children (six males, three females; mean age 9.8 years, SD 2.3 years) with ADHD participated. All underwent iodobenzamide (123I IBZM) brain SPECT within 2 hours following intravenous injection of 123I IBZM before and 3 months after methylphenidate therapy. A semiquantitative approach was used to generate indices of specific D2 receptor binding in the basal ganglia. Specific binding ratios at baseline were higher than the previously reported specific binding values obtained in studies using young healthy adults. D2 availability reduced significantly (paired t-test,p<0.05) as a function of methylphenidate therapy in patients with ADHD in all four regions of the striatum. When the relation between therapy response and D2 availability was investigated, we observed that the higher the baseline D2 levels were, the higher the response rate was (detected as the percentage reduction of hyperactivity scores and Conners Teacher Rating Scale scores), while no such trend was observed between the initial D2 binding levels and the response in attention-deficit scores. Results indicate that in non-drug treated children with ADHD, higher D2 receptor availability is observed at baseline which is down-regulated back to reported near-normal values after methylphenidate therapy. The effect of methylphenidate on D2 receptor levels in patients with ADHD is similar to that observed in healthy adults; a down-regulation phenomenon within 0 to 30%. In addition, initially higher values of D2 availability seem to indicate better response to methylphenidate therapy in ADHD.