Inclusion Criteria:

1. Willing and able to give written Informed Consent
2. Diagnosed with bilateral idiopathic PD
3. Diagnosis of PD > five years, using diagnostic criteria from core assessment program for surgical interventional therapies CAPSIT (1999)
4. Males/females between 50 and 65 years
5. Women must be postmenopausal, with last menstrual period being over five years ago
6. Male patients must agree to use barrier contraception for at least 12 months following ProSavin administration if they and their partner is of child-bearing capacity
7. Response to L-DOPA where an increase in dose is unacceptable to the patient due to potentiating the fluctuations in motor functions
8. Hoehn and Yahr stage 3 and 4
9. UPDRS (Part III) of between 20 and 60 in the "OFF" state
10. Stable dosing of PD medication, including L-DOPA, for six weeks prior to surgery
11. Positive response to dopaminergic therapy as defined by a 50% improvement in UPDRS (Part III) between the "OFF" and "ON" states
12. Presence of motor fluctuations
13. Willing to have current treatment withdrawn for up to 24 hours prior to surgery therefore being in an "OFF" state for surgery
14. Willing to have their L-DOPA dosage reduced/withdrawn at the discretion of the principle investigator (PI) at regular intervals following surgery to allow assessment of ProSavin in the absence of concomitant anti-parkinsonian medication
15. Affiliated with the French social security health care system

Exclusion Criteria:

1. Major surgery within the 28 days prior to enrolment
2. Severe disabling dyskinesias > 51% of the day as defined by the UPDRS (Part IV)
3. History of psychosis or current treatment with dopamine blocking agents of any kind
4. Severe depression as defined by a BDI score of >16. Any treatment for depression should be limited to seretonergic therapies and those that do not target the dopaminergic pathways
5. Prior treatment with tolcapone, due to its ability to modify dopaminergic pathways within the brain
6. History of epilepsy or any other co-morbid condition that the Investigator believes presents an unacceptable health risk to the patient in conjunction with the procedures in this protocol
7. Life-threatening illness unrelated to PD
8. History of stereotactic or other surgery for the treatment of PD
9. Premenopausal women
10. Alcohol or other substance abuse
11. Clinically significant laboratory test abnormalities, including full blood count, chemistry panel, liver function tests, electrocardiogram (ECG), Chest X rays
12. Any contraindication for undergoing an MRI scan of the head
13. Intercurrent illness or infection 28 days prior to enrolment
14. Abnormal MRI findings such as mega cisterna, septum pellucidum, signs of severe cortical or subcortical atrophy, brain tumours, vascular diseases, trauma or areteriovenous malformations (AVM)
15. Prior regular exposure to neuroleptic agents
16. History of treatment with any agent that may induce PD or PD symptoms within the last three months prior to enrolment
17. Contraindications to use of anaesthesia
18. Treated with dopaminergic antagonists six months prior to screening
19. Concurrent anti-retroviral therapy that would inactivate the investigational agent
20. History of any investigational agent within 28 days prior to ProSavin administration
21. Participation in a prior gene transfer therapy study
22. Enrolment in any other clinical study, for any condition, including those relating to PD, throughout the duration of the ProSavin study