OBJECTIVE: To find predictors of cognitive decline and quality of life one year after bilateral subthalamic nucleus stimulation (STN DBS) in Parkinson's disease (PD). METHODS: A total of 105 patients were evaluated with a comprehensive neuropsychological assessment before and 12 months after surgery. A control group of 40 PD patients was included to control for effects of repeated testing and disease progression. We determined individual changes in cognition, mood and quality of life using a statistical method that controls for multiple comparisons. We performed logistic regression analyses to assess predictors of cognitive changes and quality of life. RESULTS: Twelve months after surgery, the improvement in motor function was 41% (UPDRS3 score in off). The STN group showed a large improvement in quality of life compared to the control group (Cohen's d=0.9). At the individual level, 32 percent (95% CI: 22 - 40) of the STN group showed a substantial improvement in quality of life. Thirty six percent (95% CI: 27 - 46) of the STN patients showed a profile of cognitive decline compared to the control group. Mood improved in 16 STN patients and declined in 16 subjects. Impaired attention, advanced age and a low levodopa response at baseline predicted cognitive decline, whereas a high levodopa response at baseline predicted improvement in quality of life. Postoperative decrease in dopaminergic medication was not related to cognitive decline. CONCLUSIONS: STN DBS improves quality of life. However, a profile of cognitive decline can be found in a significant number of patients. Levodopa response, age and attention at baseline are predictors of cognitive and psychosocial outcome.