A 24-week placebo-controlled parallel group multicentre trial to study the safety and efficacy of memantine in patients with dementia associated with Parkinson's disease and dementia with Lewy bodies. It is hypothesized that memantine will be safe and well tolerated, and more effective than placebo.

Inclusion Criteria:

• a diagnosis of Parkinson's disease (Larsen and Dupont, 1994) and dementia (DSM IV(1987; 1994), or Dementia with Lewy bodies (McKeith et al. Neurology 2005)
• mild-to-moderate or moderate dementia (i.e. MMSE 12-26, inclusive)
• the subject has given a written informed consent
• the subject is able and willing to comply with the study procedures and has a reliable caregiver (i.e. relative or nurse/nurse assistant who sees the patient at least weekly)

Exclusion Criteria:

• other brain disease of sufficient severity to cause dementia
• mental retardation
• terminal illness with life expectancy shorter than 6 months
• recent major changes in health status
• known epilepsy or previous convulsive seizure
• major depression
• severe dementia as defined by a Mini-mental State Examination score of 12 or lower
• moderate to severe renal impairment (i.e. serum creatinine > 1,5 upper limit normal (ULN) or creatinin clearance < 40ml/minute/1,73 m2
• moderate or severe heart disease (NYHA III-IV)
• moderate or severe pulmonal disease
• moderate to severe hepatic impairment (bilirubin or transaminases > 2 times ULN
• women of childbearing potential (i.e. not post-menopausal and not taking contraceptive
• the subjects is lactating
• any laboratory value(s) exceeding the limits of normality if deemed to be clinically relevant by the study physician
• known allergies to the investigational product