This is an open-label (all people know the identity of the intervention), multi-center (when more than one hospital or medical school team work on a medical research study), single arm study of multiple doses of long-acting methylphenidate in participants with ADHD. The study will consist of 2 phases; a screening phase and a treatment phase. The duration of participation in the study for an individual participant will be up to 12 weeks. Participants will be given 18 milligram (mg) of long-acting methylphenidate daily in the morning and titrated up (slow increase in drug dosage guided by participant's responses) to 36 mg per day (mg/day) on Day 8. Depending on response, tolerability and clinician's judgment, the dose could be escalated to the next dose level of 54 mg/day on Day 28 to a maximum of 72 mg/day on Day 56, until each participant achieved optimal dose. Participant's safety will be monitored throughout the study.