Background Attention deficit hyperactivity disorder (ADHD) is one of the most common mental disorders during childhood, characterized by the core symptoms of hyperactivity, impulsivity and inattention and puts great burden on children themselves, their families and the society. Osmotic release oral system methylphenidate (OROS-MPH) is a once-daily controlled-release formulation developed to overcome some of the limitations associated with immediate-release methylphenidate (IR-MPH). It has been marketed in China since 2005 but still lacks data from large-sample clinical trials on efficacy and safety profiles. The aim of this study was to evaluate the effectiveness and safety of OROS-MPH in children aged 6 to 16 years with ADHD under naturalistic clinical setting. Methods This 6-week, multi-center, prospective, open-label study enrolled 1447 ADHD children to once-daily OROS-MPH (18 mg, 36 mg or 54 mg) treatment. The effectiveness measures were parent-rated Inattention and Overactivity With Aggression (IOWA) Conners I/O and O/D subscales, physician-rated CGI-I and parent-rated global efficacy assessment scale. Blood pressure, pulse rate measurement, adverse events (AEs) and concomitant medications and treatment review were conducted by the investigator and were served as safety measures. Results A total of 1447 children with ADHD (mean age (9.52 +/- 2.36) years) were enrolled in this trial. Totally 96.8% children received an OROS-MPH modal dose of 18 mg, 3.1% with 36 mg and 0.1% with 54 mg at the endpoint of study. The parent IOWA Conners I/O score at the end of week 2 showed statistically significant (P <0.001) improvement with OROS-MPH (mean: 6.95 +/- 2.71) versus the score at baseline (10.45 +/- 2.72). The change in the parent IOWA Conners O/D subscale, CGI-I and parent-rated global efficacy assessment scale also supported the superior efficacy for OROS-MPH treatment. Fewer than half of 1447 patients (511(35.3%)) reported AEs, and the majority of the events reported were mild (68.2%). No serious adverse events were reported during the study. Conclusion This open-label, naturalistic study provides further evidence of effectiveness and safety of OROS-MPH in school-aged children under routine practice. Chin Med J 2011;124(20):3269-3274