<qa>  What is the most common psychiatric condition?Obsessive-compulsive disorder OCD is a relatively common psychiatric condition with an estimated lifetime prevalence of 2-3 of the general population. It is generally characterized by a chronic course leading to a profound impairment in psychosocial functioning and to a marked deterioration in quality of life. Today, the well-established efficacy of antidepressants, acting preferentially by blocking serotonin reuptake, in addition to psychological treatments, have considerably changed the poor prognosis of the illness. However, both conventional therapeutic approaches failed to substantially alleviate obsessive-compulsive symptoms in 20-30 of cases. Deep brain stimulation, as a reversible and adjustable surgical procedure, has recently been introduced in the field of OCD, primarily targeting either the ventral striatum VS or the subthalamic nucleus STN and leading to an approximately 40 or greater reduction in clinical symptom intensity from baseline levels in severely chronic and resistant forms of OCD. These promising findings lead to propose a comparison of the efficacy, safety and tolerability of DBS choosing either the VS or STN as brain target by conducting a multicenter, parallel-group, randomized, single-blind trial over a 13-month follow-up period. For this purpose, a total population of 28 OCD patients who meet the currently used operational criteria for defining therapeutic resistance will be recruited. The surgical procedure will consist in the implantation of stimulation electrodes with four contacts on each lead, which are stereotactically and bilaterally implanted into the targeted brain structure under local anesthesia. Per-operative, single-unit electrophysiological recordings of the neuronal activity will be performed using five parallel microelectrodes and serving as guide for the implantation of both definitive electrodes. They will be connected to a battery powered pulse-generating device five days later under general anesthesia. Thereafter, psychiatric assessments including both the Y-BOCS Yale-Brow Obsessive-compulsive scale and PI Padua Inventory for measuring OCD symptom severity, the BAS Brief Anxiety Scale, MADRS Montgomery and Asberg Depression rating Scale and HAD Hospital Anxiety and Depression Scale for determining anxiety andor depressive symptom intensity, and the CGI Clinical Global Impression rating scales for evaluating global symptom severity and treatment response will be performed every 3 months beyond the one-month postoperative free-stimulation period. This will be coupled with a large battery of neuropsychological tests exploring all executive functions in combination with precise medical records of side effects for appreciating safetytolerability of DBS. A cost-effectiveness analysis, as a formal method of comparing DBS and classical therapeutic strategies with regard to their respective resource utilization costs and outcomes effectiveness will also be carried out. Therefore, the present study may contribute to highlight the special interest of DBS, as an effective, safe, well-tolerated and cost-relevant surgical approach for the management of resistant OCD. 