In his original essay on the shaking palsy (1817) James Parkinson remarked that the "senses and intellect were uninjured." Thus, it was only in later years that the complexity ofparkinsonism and in particular Parkinson's disease with dementia was recognised. Cognitive impairment in Parkinson's disease is common and is estimated to affect more than forty per cent of patients with disease onset after age 65. Recent studies suggest that pathology of Parkin son's disease now ranks second to Alzheimer's disease as the commonest sub strate of dementia in elderly patients. The condition is...