This is a prospective, non-comparative study. Each patient will be treated for 10 weeks including 6-week titration phase and 4-week maintenance phase. After an initial baseline evaluation, patients currently receiving IR-Methylphenidate (IR-MPH) therapy will switch to receiving OROS-methylphenidate once daily. Patients receiving IR-MPH =15 mg per day will switch to receive 18 mg once daily OROS-methylphenidate. For patients on IR-MPH daily dosage >15 mg and =30 mg, the initial dose of OROS-methylphenidate will be 36 mg once daily. Other patients receiving IR-MPH higher than 30 mg per day, will switch to receive 54 mg once daily OROS-methylphenidate. During the 6-week titration phase, those patients who do not achieve the criteria of "Optimal Response" will be titrated by biweekly increase to next dose level (36 mg per day, and then 54 mg per day). The maximum dose of OROS-methylphenidate per day is 54 mg as package insert indicates. However, dose decreases are allowed if clinically intolerable adverse events emerge. At the end of 6-week titration phase, the final titration dose should be maintained for the last 4 weeks of the trial regardless of the optimal response. In summary, all patients will attend bi-weekly clinic visits for the first 6 weeks (visit 2 to 4) and monthly clinic visits for the subsequent 4 weeks (visit 5). Patients will receive 18 mg or 36mg or 54 mg once daily OROS-methylphenidate for 10 weeks.