The objectives of this study were to evaluate the effect of methylphenidate on attention deficit hyperactivity disorder (ADHD) and the comorbidity of the disorder, using child attention problem checklist to 56 (male: 38, female: 18) patients from, March 992 to February 1993. The results were as follows: Among 56 subjects, ADHD alone were 20 (335.71%) subjects, and with one additional diagnosis were 31 (55.35%) subjects and with two additional diagnosis were 5 (8.93%) subjects. There was significant improvement on symptoms in the scores rated by teachers in 7th and 28th day after menthlphenidate administration compared to baseline score (P<0.05) and 28th day score showed significant improvement compared to 7th day score (P<0.05) and 28th day score showed significant improvement compared to 7th day score (P<0.05). There was significant improvement on symptoms in the scores rated by parents in 28th day after methylphenidate administration improvement compared with 7th day score. In single administration of methylphenidate in Sunday morning score compared to afternoon score (P<0.05). In the administration of significant improvement on symptoms compared with the Sunday morning rating score of parents (P<0.05) and the 28th day comparison was also showed significant improvement on symptoms in the scores rated by teachers compared with the scores rated by parents on symptoms (P<0.05). Among group comparison, all groups showed significant improvements (P<0.05) except conduct disorder & oppositional defiant group.