<b><i>Background:</i></b> Since the introduction of subthalamic nucleus deep brain stimulation (STN DBS), many clinical studies have shown that this therapy is safe and effective in the short and medium term. Only little is known about long-term results. <b><i>Objectives:</i></b> To provide an analysis of motor and cognitive outcome 10 years after STN DBS. <b><i>Methods:</i></b> In this observational cohort study, we report on the motor and cognitive outcome in a cohort of 26 Parkinson's disease patients who were prospectively followed up for 10 years after STN DBS surgery. <b><i>Results:</i></b> In the early post-operative phase, improvement in the Unified Parkinson's Disease Rating Scale (UPDRS) III (10.6, p < 0.01) and IV (2.5, p < 0.01) was seen as well as a 32% reduction in levodopa equivalent dose (p < 0.01). After 5 years, a worsening of the motor performance was observed. The worsening of motor performance was mainly due to a deterioration in bradykinesia (12.4 ± 4.6, p < 0.05) and axial symptoms (6.9 ± 2.8, p < 0.01). Memory function seemed to improve in the short term, but there was a significant decline between 1 and 5 years after surgery (p < 0.01). Mood remained relatively stable during follow-up, and one third of the patients showed impulsive behaviour after surgery. <b><i>Conclusions:</i></b> The motor performance of patients showed deterioration over time, due to an increase in bradykinesia and axial symptoms.