Inclusion Criteria:

1. A diagnosis of Parkinson's disease according to UK Parkinson's Disease Society Brain
Bank Criteria. These criteria are in standard use throughout the NHS in the UK and
were supported by the NICE guidelines.

2. People with mild dementia associated with PD, where the patient and/or their family
have become aware of cognitive with or without behavioural symptoms that are causing
functional impairment. "Dementia" will be defined according to recently published
Movement Disorder Society Task Force criteria for dementia associated with
Parkinson's Disease and "operationalised" using the Addenbrooke's Cognitive
Examination (ACE-R). The ACE-R permits some description of the dementia profile and
also quantifies global impairment. It is increasingly used by clinicians in the UK
to identify demented subjects, is relatively quick to perform (15 minutes or so),
requires no specific training and produces a total score (0-100), from which the MMSE
(0-30) can also be extracted. Participants will have an ACE-R of 88 or less. If
this criterion is met, subjects will be further assessed using the Mattis Dementia
Rating Scale (DRS-2). An age- and education-corrected total DRS-2 score of less than
8 but greater than 6 (corresponding to between the 6th and 28th percentile) will be
used to define "mild" dementia".

3. Community-living and a spouse, close relative or well established carer to accompany
the subject to act as an informant.

4. Where relevant, women of child bearing potential must be using adequate contraception
for duration of study.

Exclusion Criteria:

1. Dementia that develops within one year of the onset of motor symptoms. The reason
for this "one year rule" is to specifically exclude participants with Dementia with
Lewy Bodies (DLB). This exclusion criterion is consistent with recommendations made
in the Movement Disorder Society Dementia Task Force Diagnostic Criteria and the
Third Report of the DLB Consortium.

2. People with such severe motor disability, or who are so impaired in their activities
of daily living from other aspects of their PD, that it would interfere with
cognitive and global assessments.

3. Severe current depressive episode. Low mood may impact upon accurate cognitive
assessment and major depression is therefore listed as a feature which, when present,
makes it impossible to reliably diagnose PDD in the Movement disorder Society Task
Force PDD Criteria. This will be operationalised using the self-completed Beck
Depression Inventory and a cut-off score of 13, as recommended by a recent Movement
Disorder Society Task Force report. The BDI score is considered robust in the face of
mild to moderate cognitive impairment.

4. Unstable significant medical co-morbidity.

5. Patient receiving an anticholinergic drug for control of parkinsonian motor symptoms.

6. Previous exposure to a cholinesterase inhibitor

7. Presence of a condition that is contraindicative to use of donepezil (including a
clinically significant cardiac conduction defect found in patient history or from
screening ECG); see SmPC (Appendix W) for details.

8. Allergy/hypersensitivity to excipients of donepezil or placebo

9. Patient receiving the N-methyl-d-aspartate antagonist memantine.

10. Previous neurosurgery for Parkinson's disease. This will apply to only a small
minority of predominantly younger cases. The main reason for this exclusion relates
to ongoing uncertainty over the potential confounding effects of deep brain
stimulation upon both mood and cognition.