The purpose of this exploratory study is to determine whether memantine can provide benefits on clinical symptoms in patients with Parkinson's Disease Dementia or Dementia with Lewy Bodies.

The objective of this exploratory study is to assess the benefits of memantine compared to placebo in out-patients with a diagnosis of Parkinson's Disease Dementia or Dementia with Lewy Bodies (mild to moderate severity) over a 6-month period. This is a multinational, randomised, double-blind, parallel-group, placebo-controlled, fixed-dose study with specific cognitive (attention, executive function, visual perception and memory), behavioural, functional and global measures.

Inclusion Criteria:

• The study population will consist of male or female outpatients at least 50 years of age with mild to moderate disease severity (MMSE 12 to 24 inclusive) according to UKPDS and DSM IV TR criteria (for PDD patients) and the third report of the DLB consortium (for DLB patients) and who have a knowledgeable and reliable caregiver to accompany the patient to all clinic visits during the course of the study.

Exclusion Criteria:

• Evidence of clinically significant active disease, evidence of other neurological disorders, and current treatment with AChEIs.