Striatal dopamine (D2) receptor availability was determined by iodobenzamide brain SPECT, before and 3 months after methylphenidate (MPH) therapy, in 9 children (mean age, 9.8 years) with attention deficit hyperactivity disorder (ADHD) examined at Gazi University, Ankara, Turkey. Specific D2 receptor binding ratios in ADHD subjects at baseline were higher than those reported in healthy young adults. Severity of ADHD behavioral symptoms was correlated with D2 receptor levels; the higher the Conners Teacher Rating Scale scores, the higher the baseline D2 availability in the putamen and caudate. D2 receptor availability was significantly reduced in all four regions of the striatum following MPH therapy. The higher the baseline D2 levels, the greater the behavioral response to MPH (as determined by improved hyperactivity and Conners Teacher Rating scores), whereas the response in attention deficit scores was not related to baseline D2 levels. D2 receptor levels in patients with ADHD may predict the behavioral response to MPH. 
COMMENT. D2 receptor levels are higher than normal in untreated children with ADHD, and are reduced to near-normal values following treatment with methylphenidate. The higher the pre-treatment D2 receptor availability, the greater the severity and number of ADHD behavioral symptoms and the better the behavioral response to therapy. These findings are in accord with previous reports of clinical predictors of response to MPH therapy in ADHD (in Attention Deficit Hyperactivity and Learning Disorders, PNB Publ, 2001) . Children who are most active are most likely to respond to MPH.