Objectives : The purpose of this study was to investigate clinical characteristics of children and adolescents with autism spectrum disorders (ASDs) using methylphenidate (MPH). Methods : Retrospective review of the charts of 79 children and adolescents with ASDs, who visited the Department of Child and Adolescent Psychiatry of Seoul National Hospital, from July 2010 to July 2011, was conducted. Changes in illness severity and improvement were measured using the Clinical Global Impression-Severity of illness (CGI-S) and Clinical Global Impression-Improvement (CGI-I) Scales. Results : We found that MPH was prescribed in 23 (29.1%) children and adolescents. Of the 23 patients on MPH, 4 patients (17.4%) were on MPH monotherapy and 18 patients (78.3%) were using risperidone concomitantly. MPH was prescribed primarily for symptoms of hyperactivity and impulsivity in ASDs patients. The mean dosage of MPH was $26.2{pm}11.1$mg/day and mean duration of treatment was $31.9{pm}28.7$ months. Mean CGI-S score improved significantly from baseline to endpoint (from $5.4{pm}0.6$ to $4.1{pm}0.9$ ; p<.01). MPH was reported to be effective in 17 patients (17/23, 73.9%), and 10 patients (10/23, 43.5%) reported side effects. Side effects included decreased appetite (4/23, 17.4%), tic (2/23, 8.6%), sleep disturbances (2/23, 8.6%), headache (1/23, 4.3%) and irritability (1/23, 4.3%). Conclusion : The results of this study demonstrate that MPH may be used effectively and safely in children and adolescents with ASDs with hyperactivity and impulsivity. Future controlled trials are needed to confirm these findings.