We investigated the moderating influence of apathy, depression and transient mood changes on executive functions under best medical treatment and under postoperative stimulation-on and -off conditions in a sample of 33 patients with Parkinson's disease (PD) after deep brain stimulation of the subthalamic nucleus (STN), 33 PD patients with pharmacological treatment only and 34 healthy controls. In comparison to clinical and healthy control groups, DBS patients showed worse executive task performance and also more severe symptoms of depression and apathy. Apathy accounted for differences in stroop interference between groups. The effects of DBS on stroop interference were explained by increased state anxiety in the -off, so that DBS STN had no significant influence on test performance. Consideration of neuropsychiatric symptoms and acute mood changes is an important aspect when evaluating neuropsychological deficits in DBS patients.