Gait disturbance and postural instability are some the most disabling symptoms of idiopathic Parkinson's disease and in late stage disease can be resistant to both medical and surgical therapies. We implanted bilateral deep brain stimulation electrodes into the pedunculopontine nucleus in two patients with advanced Parkinson's disease. We demonstrate for the first time that low frequency (20-25 Hz) stimulation of this nucleus significantly improves gait dysfunction and postural instability in both the 'on' and 'off' medication states. Their combined total Unified Parkinson's Disease Rating Scale score improved by 53% and motor score by 57%. No procedure or stimulation-related complications were observed. If these findings are replicated in a larger number of patients, pedunculopontine nucleus stimulation may provide the means to alleviate these disabling and otherwise treatment-resistant symptoms of advanced Parkinson's disease.