The severe, cortical, cholinergic depletion accompanying Parkinson's disease (PD) is considered as a highly probable correlate of cognitive and behavioural dysfunction. Recent studies have demonstrated that cholinesterase inhibitors (notably rivastigmine) are beneficial in patients suffering from dementia associated with PD (PDD). However, the primary efficacy variables used in such work came from scales designed for Alzheimer's disease (AD), even though the cognitive symptoms in PD and AD dementia do not overlap completely. The aim of the present study (a double-blind, placebo-controlled clinical trial) was to determine the utility of the Mattis dementia rating scale - the most commonly used scale in PD patients - to assess the efficacy of a 24-week rivastigmine treatment.