Electrical stimulation of the subthalamic nucleus is an effective treatment for the motor symptoms of Parkinson's disease. While most patients who undergo this procedure do not appear to suffer behavioral side effects, a minority experience cognitive or emotional deficits, and longitudinal studies have reported declines; however, the measures of cognitive function used have been limited. One explanation for the possible disturbance of cognitive functions is that electrical stimulation of the subthalamic nucleus disrupts the normal flow of information within cortico-striatal loops involving prefrontal, associative, or limbic cortex. We wished to assess the effect of high frequency electrical stimulation of the subthalamic nucleus in Parkinson's disease patients while they performed a comprehensive neuropsychological test battery. We selected cognitive tasks known to test the function of different cortical areas, including tests of executive function, cognitive flexibility, attention, memory, language and visual perception. Patients were tested on two separate days, with the stimulators turned on or off. Test scores were also compared to preoperative performance. In our sample of 15 patients without dementia or major depression there was no deterioration on any cognitive test as a result of stimulation. We conclude that electrical stimulation of the motor subthalamic nucleus does not cause appreciable declines in cognitive function in well-selected patients.