Cognitive impairment is frequent in patients with Parkinson's disease (PD), and can range from mild deterioration to dementia. Recently a contribution of Alzheimer's disease for the cognitive dysfunction in PD has been proposed, whereas the presence of tau protein and amyloid was recognized. Clusterin/ApoJ is a protein involved in the deposition of beta-amyloid and in its neurotoxicity. In this study we aimed to investigate the clusterin/ApoJ's plasma levels in patients with PD to assess its potential role in fisiopathogenetic cognitive impairment.