We report here the results of an open prospective study in 9 patients suffering from severe Parkinson's disease with on/off fluctuations and restricted off-period mobility, who underwent bilateral implantation of stimulating electrodes in the internal pallidum. At 3-month follow-up, the total Unified Parkinson's Disease Rating Scale (UPDRS) motor score in the medication-off state was reduced from 54.1+/-14.8 to 23.9+/-11.7 (44.2%) when stimulation was turned on. Comparison of UPDRS subscores revealed significant improvements for tremor, rigidity, bradykinesia, gait and posture, and dyskinesias. The results of the clinical scoring could be confirmed by significant changes in the quantitative assessment of hand function and walking. Bilateral pallidal stimulation reduced the amount and severity of on/off fluctuations. Additional follow-up at 6 months (n=6), 9 months (n=6), and 12 months (n=4) did not show a decline in effectiveness of stimulation. There was no permanent morbidity associated with the procedure. A subtle reduction of verbal fluency, which was not evident to the patients, was the only cognitive side effect of the procedure in neuropsychological testing. Chronic bilateral high-frequency stimulation of the internal pallidum seems to be a neurologically safe and highly effective treatment for "off" symptoms, dyskinesias, and motor fluctuations in advanced stages of Parkinson's disease.