Introduction: Improvements in motor scores are well described in subthafamic deep brain stimulation (DBS) but changes in cognition and mood are less well known. Material and methods: We report mood swings related to changes in parameters of stimulation in one patient with subthalamic DBS free of antiparkinsonian medication. Results: 51 year-old patient with a 17 year history of L-dopa responsive parkinsonism with major on-off fluctuations of the yo-yoing type underwent bilateral pallidal DBS for two years without success. Relocation of the electrodes one year later did not bring any furtber improvement. After 2 years, the patient underwent bilateral contemporaeous subthalamic DBS (2.5 V. monopolar. 185 Hz, 90 msec duration on both sides) with major improvement (preoperative off/on Motor UPDRS 3818 was improved to 5. The patient did not require any antiparkinsonian medication went rid of any fluctuation. Switching of the stimulator *on* was first followed by an elation in mood, lasting for most of the 2 following months, with a kind of euphoria and hypomanic activity that the patient was not able to Perform since more than IO years (walking, calling friends, running, and he even took a free activity in a school). During this pericde of follow-up, adjustments of stimulation parameters was each time followed by an urge to cry lasting for I-2 days, then progressively replaced by elation and euphoric mood. At 3 months. the patient was switched off in order IO tested the effect of stimulation. After this test, he was in a sad mood for more than 3 days and did never recover since the euphoria that he presented for the first 2 months. He reported instead paroxysmal repetitive episodes of a feeling of tightness in the lower sternal region, lasting for a few minutes, followed by few hours of euphoria and impression of well being just thereafter. He first was afraid from these episodes to occur, but is now expecting them, because of the intense pleasure he would feel just after the sternal feeling. Conclusion: Subthalamie DBS may not only be followed by major improvements in motor score, but bipolar swings in mood can occur, which are largely influenced by changes in parameters of DBS stimulation. This allows us IO suspect a major role for the subthalamic nucleus in the genesis of bipolar mood disorders. Supported by Swiss National Science Fundation N 3 I-53006.97