Disturbance of posture, gait and balance usually appear several years after Parkinson's disease (PD) onset. These axial signs, generally resistant to conventional drug and non-drug treatment, can generate a loss of autonomy and traumatic complications. Improvement of these axial signs by deep brain stimulation (DBS) of the pedunculo-pontin nucleus (PPN) has been described for a small number of patients. The objectives of the present prospective monocentric pilot study are to determine the optimal stimulation parameters of PPN-DBS, to evaluate the long term tolerance and the effect of PPN-DBS on axial signs and quality of life in five patients with severe PD. The use of detailed clinical rating scales and quantitative computer-based gait analysis will allow to identify the optimal stimulation parameters and relevant clinical efficacy parameters, mandatory for the design of further comparative large-scale studies.