Therapeutic management of gait disorders in very advanced Parkinson's disease (PD) patients can sometimes be disappointing, since dopaminergic drug treatments and subthalamic nucleus (STN) stimulation are more effective for limb-related parkinsonian signs than for gait disorders. Gait disorders could be also partly related to noradrenergic system impairment, pharmacological modulation of both dopamine and noradrenaline pathways could potentially improve the symptomatology. The investigators have demonstrated using an open label study on 17 advanced PD patients that chronic, high doses of methylphenidate (MPD) improved gait, freezing of gait, motor symptoms and attention in the absence of L-Dopa and increased the intensity of response of these symptoms to L-Dopa (Devos et al., 2007). The investigators aimed to confirm their results using a randomized, double-blind, placebo-controlled, parallel-group, multicentric trial. The investigators will assess the clinical value of chronic, high doses (1 mg/kg/day) of MPD vs placebo in 88 non demented PD patients suffering from severe gait disorders with freezing despite their use of optimal dopaminergic doses and eventually STN stimulation parameters. Efficacy will be assessed directly and on video in the absence of L-Dopa and again after acute administration of the drug, both before and after a 3-month course of MPD, using Stand Walk Sit test (primary criteria), the "Freezing Of Gait trajectory", RGSE scale, the UPDRS scores, the dyskinesia rating scale, Achiron scales and using auto-questionnaires of Giladi, ABC scale and PDQ 39. Attention will be assessed using reactions times. Drowsiness will be assessed using Epworth and Parkinson's disease Sleep Scales. Apathy and depression will be monitored with Lille Apathy Rating Scale, MADRS, BPRS, MINI and psychiatric interview. Cardiologic and general tolerance will be also monitored. This study could lead to propose methylphenidate with a good efficacy/ risk balance in advanced PD patients suffering from severe gait disorders with freezing of gait, drowsiness and attention deficit.