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Patients with Parkinson’s disease (PD) often suffer from non‐motor symptoms, which may be caused by serotonergic dysfunction. Deep Brain Stimulation (DBS) in the subthalamic nucleus (STN) may also influence non‐motor symptoms. The aim of this study is to investigate how the cerebral 5‐HT system associates to disturbances in cognition and mood in PD patients with DBS‐ STN turned on and off. We used psychological tests and questionnaires to evaluate cognitive function and the effects on mood from turning DBS‐STN off. We applied a novel PET neuroimaging methodology to evaluate the integrity of the cerebral serotonin system. We measured 5‐HT1BR binding in 13 DBS‐STN‐treated PD patients, at baseline and after turning DBS off. Thirteen age-matched volunteers served as controls. The measures for cognition and mood were correlated to the 5‐HT1BR availability in temporal limbic cortex. 5‐HT1BR binding was proportional to working memory performance and inverse proportional to affective bias for face recognition. When DBS is turned off, patients feel less vigorous; the higher the limbic and temporal 5‐HT1BR binding, the more they are affected by DBS being turned off. Our study suggests that cerebral 5‐HTR binding is associated with non‐motor symptoms, and that preservation of serotonergic functions may be predictive of DBS‐STN effects.