This is a 41-week trial composed of two consecutive parts (Part A, 14 weeks and optional Part B, 27 weeks) to evaluate the efficacy of levetiracetam in controlling levodopa induced dyskinesias in adults with Idiopathic Parkinson's Disease, without negative impact on the benefit on the motor function of the antiparkinsonian treatment.

Inclusion Criteria:

1. Male / female adult, 30 years old or more;

2. Subject suffering from Idiopathic Parkinson's Disease, diagnosed according to the UK Brain Bank Criteria (see Appendix 14.8) (1) , since a minimum of 3 years, and stabilized with regard to the motor function (ratio .on./.off. over the day);

3. Subject treated with levodopa whether or not combined with any other antiparkinsonian drugs (except apomorphine) for minimum one year;

4. Subject experiencing during .on. periods, despite several attempts of treatment adjustment, persistent troublesome dyskinesias:

   1. with a minimum score of 2 on item 33 of the UPDRS-Part IV (moderately disabling dyskinesias);
   2. with a minimum score of 2 on item 32 of the UPDRS-Part IV (dyskinesias present during more than 25% of the waking day).

Exclusion Criteria:

1. Dyskinesias that cannot reliably be characterized on a regular basis by the subject himself/herself (more than 5 .errors. on DRC per 24 hours after repeated training);
2. Subject currently treated with apomorphine or neuroleptic drugs either typical or atypical such as clozapine, olanzapine or risperidone;
3. Subject currently treated with dopamine antagonists such as but not limited to metoclopramide, alizapride or phenothiazine derivatives (e.g. metopimazine). If necessary, the use of the dopamine antagonist domperidone is allowed.
4. Subject Who had undergone or who was scheduled for functional neurosurgery for Parkinson's disease during the trial period;