Background and Purpose: Cerebral amyloid angiopathy (CAA) is one of the causes of vascular cortical dementia seen in 78 -100% of patients with Alzheimer's disease (AD) (Attems, 2005). CAA is associated with cerebral microbleeds (Dierksen, 2010) which recently are of special interest as they can play an important role in the pathophysiology of AD (Cordonnier, 2011) and can be involved with cognitive dysfunction (Jellinger, 2008). Though pathology of dementia with Lewy body consists of the combination of that of both Lewy body and AD, it is unclear whether or not it is applicable to that of Parkinson's disease (PD). Our aim was to elucidate the role of CAA to cognitive dysfunction in PD by an MRI study.Method: We retrospectively analyzed data from patients with PD (without dementia: n=46, with dementia: n=6) and AD (n=29) who had detailed diagnostic evaluation including T2*-weighted gradient-echo brain MRI at Kyoto City Hospital from January 1st in 2011 to August 31st in 2013. MRI imaging was performed on 1.5 T scanners. Microbleeds were defined as well-delineated focal areas of low signals on T2*-weighted axial image in 2-5 mm diameter, and were judged throughout brain by a neurologist who was blinded to the clinical information of the patients. The existence of microbleeds in MRI as categorical variable was compared by means of χ2 test. Results: AD patients had significantly higher ratio (58.6%, p=0.03), and PD with dementia (PDD) had higher ratio with marginal significance (66.7%, p=0.10), of presence of microbleeds as compared with PD without dementia (32.6%). Discussion: The present study using MRI showed that cognitive dysfunction of not only AD but also PDD patients can be associated with microbleeds which topographically relate to the micro-vessel damage as CAA.