• The primary outcome measures will be the change in the gait composite score between the MPD and Placebo groups, at week 12 after completion of each study arm, immediately before and at the end of the cross-over period. [ Time Frame: At week 12 and 27 ] [ Designated as safety issue: No ]
  

• Secondary outcome measures include changes in gait diary (number of freezing episodes and falls), static posturography, FOGQ (total score), MADRS, EQ-5D, and the ESS between days 1 and 56, that is, at the end of week 12 for each study arm. [ Time Frame: At week 12 and 27 ] [ Designated as safety issue: No ]
  

There are a total of six clinic visits involved in this study. All study-related assessments will take place first during the practically defined off period, that is, in the morning after at least 12 hours from the last dose of any antiparkinsonian medication, followed by a repeat assessment once the "on" state is clearly identified by the patient and examiner (approximately 30 to 60 minutes after taking your Parkinson's medication). This "practically defined off period" state is considered the desired state on which to report motor changes for any currently available or experimental intervention in PD. Patients will be off his or her Parkinson's medication for no more than 14 hours for each of the assessments. Studying patients in the off-period is the most widely used manner in which the value of new therapies can be fully measured.

During the first visit, patients will be "randomized" into one of the study groups described below. Neither the participant nor the researcher conducting this study will choose what group he or she will be in.

Participants will receive either placebo or a dose of 1 mg/kg of Methylphenidate capsules divided into three doses (at 8 am, 12 noon, and 4 pm). An increase in medication over a four week period will be used until a target dosage is reached, which may range from 5 to 8 10-mg capsules per day.

A measure of balance will be taken during both patient's "off" and "on" motor states during each of the study visits. Patients will be asked to stand on a force plate (a piece of equipment that measures your balance and is located in the floor) for thirty seconds in a total of four conditions. The four sessions will be carried out as follows: (1) with eyes open while standing on firm surface, (2) with eyes closed while standing on firm surface, (3) with eyes open while standing on foam surface, and (4) with eyes closed while standing on foam surface. These sessions will be in random order and will be repeated up to 4 times.

During all the study visits, the physician will ask patients to perform some physical tests during which the Unified Parkinson's Disease Rating Scale (UPDRS) and the Hoehn and Yahr (H&Y) will be used to assess the severity of the patient's Parkinson's disease. Each assessment will take place during both patient's "off" and "on" motor states. Instruments that will also be used are the self-administered Freezing of Gait Questionnaire (FOGQ), to evaluate walking difficulties, and the Gait-Falls diary, to document all indoor and outdoor freezing, tripping, and falls. To assess changes in mood, patients will also complete the following instruments: the Montgomery-Asberg Depression scale (MADRS), the 15-item Geriatric Depression Scale (GDS-15) and the 20-item Zung Self-Rating Depression scale (Zung). To assess quality of life and activities of daily living, patients will complete the EQ-5D Health Questionnaire. Finally, patients will be asked to complete the Epworth Sleepiness Scale (ESS) to document any changes in sleep patterns.

After the first visit, patients will begin a four week increase in medication in order to reach a target dose. At the fourth visit, patients will receive the opposite treatment of what was received the first time and another four-week increase in medication will take place. If Methylphenidate was given at the first visit, patients will receive placebo on the third visit and vice versa. There will be a three week period where patients will not take the study medication (either Methylphenidate or placebo), between the third and fourth visit. At all study visits, during both patient's "off" and "on" motor states, patients will have balance testing. The physician will evaluate the severity of patient's disease using the UPDRS and the H&Y. Patients will be asked to complete the FOGQ and the Gait-Falls diary.