This is a randomised, double-blind, placebo-controlled, multicentre study comparing the effect of bilateral DBS of the GPi with placebo in patients with invalidating primary generalized dystonia. After inclusion in the study (see in/exclusioncriteria), a preoperative formal baseline assessment is performed. Within six weeks after the inclusion the patients undergo the stereotactic implantation of the stimulation equipment in the GPi. Immediately after surgery the patients are randomly allocated to direct stimulation (treatment group) or delayed stimulation (control group), in which the stimulator will se switched on 0 Volt before discharge from the hospital by an independent neurologist or neurosurgeon. The patients in both groups are able to control whether the stimulation is active (>0 Volt) or not. In this way the patients and the assessors are blinded. In each centre the postoperative care of the patients will take place according to a stratified stimulation parameter protocol which assures comparable information in both groups. Six months postoperatively, the first formal outcome assessment is performed by the same blinded assessor. After completion of this assessment the stimulators in the control group are activated by an independent neurologist or neurosurgeon. From this moment, all 24 patients have active DBS. However, the patients and blinded assessor remain unaware of the result of the randomisation. All patients are followed for another six months after which the second outcome assessment is made by the same assessor. Four Dutch centres experienced in DBS have agreed to participate in the study. It is estimated that each centre will include at least 4 patients. Because these centra are geographically spread over the country patients can be operated and controlled nearby.