Gait and balance disorders represent the main motor disability in advanced Parkinson's disease. These symptoms are less or unresponsive to levodopa treatment and are considered to be a contraindication for deep brain stimulation of the subthalamic nucleus. Falls and freezing of gait are responsible for high morbidity (fractures, residential health care) and increased significantly mortality. The pathophysiology of gait and balance disorders is still poorly understood, but recent data obtained in animals and humans suggest that a degeneration of cholinergic neurons of the pedunculopontine nucleus (PPN), within the mesencephalic locomotor region, could play a crucial role. In line with this hypothesis, low-frequency stimulation of the pedunculopontine area, thought to increase the activity of the remaining cholinergic PPN neurons, has been proposed to alleviate gait and balance disorders in advanced PD patients. Here, the efficacy of deep brain stimulation of the mesencephalic locomotor region will be tested in 12 PD patients in a randomized, double-blind, cross-over, controlled study.