Background: Attention-Deficit/Hyperactivity Disorder (ADHD), characterized by inattention, hyperactivity, and impulsivity, is among the most common neuropsychiatric disorders worldwide (5-10% at school age) and in Taiwan (7.5%). The DSM-IV ADHD consists of three subtypes: predominantly Inattentive (ADHD-I), predominantly Hyperactivity/Impulsivity (ADHD-H), and Combined subtypes (ADHD-C). Although the DSM-IV field trials indicate that the current subtypes differ significantly on variables such as age of onset, gender ratio, and level of social and academic impairment, little has been addressed whether the DSM-IV subtypes differ neuropsychologically. Numerous researches have proved that stimulants (methylphenidate is the only stimulant in Taiwan) are the most effective medication in treating ADHD. Although the efficacy of methylphenidate on behavioral measures has been established in Taiwan, there is no information about neuropsychological measures.