This is an open label, multicentre extension study to trial 42603ATT3002. Patients, who were enrolled to trial 42603ATT3002 must have had a diagnosis of ADHD with some symptoms already present at the age of 7 and continuing in adulthood. The patients must have had at least mild to moderate symptoms of ADHD. In trial 42603ATT3002 the efficacy and safety of three fixed dosages of the study drug OROS methlyphenidate (MPH) was compared with placebo in adult subjects with ADHD in a double-blind phase. This double-blind phase was followed by an open-label phase, which was designed to assess the safety and tolerabiltiy of the study drug OROS methylphenidate in these subjects, when they got a dose, which was optimal to control their symptoms. In trial 42603ATT3004 patients are enrolled who completed the open-label phase of trial 42603ATT3002. Trial 42603ATT3004 is designed to assess the long-term safety and tolerabilty of OROS methylphenidate over a period of 52 weeks at a dose, which is optimal to control the symptoms. Patients may enter the extension study 42603ATT3004 at Visit 8 (End-of-Phase Visit, Open Label) of the 42603ATT3002 study as long as the patient has given informed consent prior to the visit being performed. Visit 8 data may then serve as baseline data for the extension study. These patients should continue treatment at the same optimal dosage of PR OROS methylphenidate as was attained during the open label phase of protocol 42603ATT3002. Patients who complete the 42603ATT3002 study, including post-study visit, may enter the extensions study up to 30 calendar days from the last dosing of open label trial medication of the 42603ATT3002 study. Patients should be titrated accordingly to their optimal dose attained in study 42603ATT3002. Patients will be assessed at regular intervals for at least 52 weeks.

Amendment: At completion of the open-label phase, patients can participate in a double-blind placebo-controlled phase, provided that they are giving written informed consent. At inclusion to the double-blind placebo-controlled phase, patients are assigned to one of two treatment arms by chance. One treatment arm will continue taking the same treatment at the same daily dose as during the open-label period. The other treatment arm will receive placebo treatment. As placebo, an inactive formulation or an empty treatment will be used. During the double-blind placebo-controlled phase, patients are asked to return to their doctor's office every other week. During these visits, physical examinations will be made, efficacy measures will be taken by means of questionnaires, that need to be completed either by the treating physician or the patient and any unwanted side effect will be recorded and treated if needed. One week after the end of the double-blind, placebo-controlled phase, patients have to return one more time to their doctor's office for the so called post-study visit, during which an additional physical examination will be performed and additional safety and efficacy parameters will be assessed.Patients will be assessed at regular intervals for at least 52 weeks. Daily oral dosages of 18, 36, 54, 72 and 90mg will be available and may be increased or decreased by 18mg increments based on clinical observations of response and tolerability for at least 52 weeks. Amendment: During the double-blind, placebo-controlled phase, patients will receive either active treatment at the previously assessed optimal dose or placebo tablets for a total of 4 weeks. Both treatments (active or placebo) are taken by mouth once per day in the morning.