The effects of subthalamic nucleus (STN) stimulation on cognition and mood have not been well established. The authors estimated cognitive and mood effects of bilateral subthalamic nucleus deep brain stimulation (STN DBS) in patients with Parkinson's disease (PD) at 6 months and 1 year postoperatively. Forty-six patients were recruited from the Movement Disorder Center at Seoul National University Hospital. Neuropsychologic tests were performed three times, before, 6 months after, and 1 year after surgery. Mean patient age was 58 and mean education duration 8 years. Eighteen of the 46 patients were men. The instruments used for assessing cognitive functions were; the Mini-Mental Status Examination (MMSE), the Trail Making Test (TMT), the Korean Boston Naming Test (K-BNT), the Rey-Kim Memory Battery, the Grooved pegboard test, the Stroop test, a fluency test, the Wisconsin Card Sorting test (WCST), and the Beck depression inventory (BDI). Of these tests, the verbal memory test, the Stroop test, and the fluency test showed statistically significant changes. The verbal memory test using the Rey-Kim memory battery showed a decline in delayed recall and recognition at 6 months and 1 year postoperatively, whereas nonverbal memory showed no meaningful change. In terms of frontal lobe function tests, Stroop test and fluency test findings were found to be aggravated at 6 months and this continued at 1 year postoperatively. Previous studies have consistently reported a reduction in verbal fluency and improvements in self-reported symptoms of depression after STN DBS. However, in the present study, Beck depression inventory (B.D.I.) was not significantly changed. Other tests, namely, MMSE, TMT, K-BNT, Grooved pegboard test, and the WCST also failed to show significant changes. Of the baseline characteristics, age at onset, number of years in full-time education, and L-dopa equivalent dosage were found to be correlated with a postoperative decline in neuropsychological test results. The correlation of motor improvement and cognitive deterioration was not significant, which suggests that the stimulation effect is rather confined to the motor-related part in the STN. In conclusion, bilateral STN DBS in Parkinson's disease did not lead to a significant global deterioration in cognitive function. However, our findings suggest that it has minor detrimental long-term impacts on memory and frontal lobe function.