The purpose of this research is to evaluate the usefulness of memantine, compared to placebo (sugar pill), for the treatment of cognitive impairment in patients with idiopathic Parkinson's disease (PD) and dementia. Memantine is used as a safe and effective treatment for patients with Alzheimer's disease. Cognitive impairment includes concentration and memory difficulties. We will look at how well this medication helps your cognitive impairment, how well you tolerate this medication (including its effects on your motor symptoms of PD) your activities of daily living, your emotions, and any medical conditions you might have. We will interview a person you choose as your "informant".

This is a randomized, placebo-controlled, parallel, double-blind 24-week prospective study of memantine at the dosage range 5-20 mg/day in 20 outpatients with idiopathic PD and dementia secondary to PD. Using the dosage escalation regimen approved for Alzheimer disease, subjects will start memantine or comparable placebo at 5 mg daily and advance 5 mg/week to 20 mg /day by week 4, with dosing at 10 mg bid. Subjects will undergo 7 clinical visits over the 6-month trial (Screen, Baseline/Week 0, and Weeks 4, 8, 14, 20, and 24). The dosage can be titrated downward in increments of 5 mg to a minimum dose of 5 mg/day in the event memantine is not tolerated at the scheduled dosages. This broad dose range is being used because 1)a favorable cognitive response may be evident at lower doses of memantine than recommended for AD and 2)adverse effects could emerge when typical AD dosing recommendations are used, as has been observed when treating PD patients with cholinesterase inhibitors. Subjects will remain on a stable dose of memantine/placebo after Week 8, unless precluded by adverse events. Ten subjects will be assigned to each treatment group. Randomization will be stratified according to whether subjects are taking a concomitant cholinesterase inhibitor. This will enable secondary group comparisons of treatment groups. Results from this initial small study will be used to evaluate the appropriateness of devising a larger-scale multi-site study of memantine for treatment of dementia in PD.

The proposed assessment schedule was designed to represent use of memantine in general clinical practice and to minimize the burdens to caregivers and patients, who have impaired mobility as well as cognitive function.

Inclusion Criteria:

1. Diagnosis of idiopathic PD, as defined by UK Brain Bank Criteria.
2. Age onset of PD > 35 years old
3. Adult men and women, current age > 50 years
4. English speaking
5. Any race or ethnic background.
6. Hoehn and Yahr Stage I-V, provided able to participate verbally in clinical assessments and travel to clinic.
7. Diagnosis of dementia secondary to PD, as defined by DSM-IV-TR.
8. Stable medical health
9. Taking stable doses for 2 months of non-excluded medications.
10. Outpatient status (may be residing in a long-term care facility).
11. Able to attend all study visits with an informed caregiver/partner who is willing to provide information on the patient's clinical status and response to treatment.
12. Presence of an informed caregiver willing to take part in weekly phone call follow-up calls for the duration of study enrollment.
13. Provision of informed consent by patient and caregiver and/or legal guardian.
14. On stable antiparkinsonian therapy for 2 months.
15. If history of major depression or anxiety disorder, must have stable symptoms and be on stable therapy for 2 months.
16. If taking antipsychotic medication, must be on stable therapy for 2 months.
17. If taking nonsteroidal anti-inflammatory medication, selegiline, or estrogen, must be on a stable dose for 30 days before study entry.
18. If taking cholinesterase inhibitors, must be on for at least 6 months and a stable dose for 2 months before randomization.

Exclusion Criteria:

1. History or evidence of neurodegenerative disorder other than PD.
2. Meets clinical criteria for Dementia with Lewy Bodies.
3. History or current evidence of epilepsy.
4. Participation in another investigational drug trial within 2 months of screening.
5. Treatment with memantine within 60 days of screening.
6. Current symptomatic Major Depressive Disorder, as based on Hamilton Depression Rating Scale Score > 17.
7. Current clinically significant hepatic, kidney disease, gastrointestinal, endocrine, or cardiovascular disease, including evidence of second or third degree heart block. [Note, patients with controlled hypertension (supine diastolic BP<95 mm Hg), complete or partial right bundle branch block, pacemakers, or deep brain stimulators may be included.].

6 months