BACKGROUND: Cerebral microbleeds (CMBs) in patients with Parkinson's disease (PD) or dementia with Lewy bodies (DLB) have not been adequately studied. OBJECTIVE: This study aims to find a difference in the total number, prevalence, and common locations of CMBs between PD and DLB and evaluate 99 mTc-ECD SPECT subtraction images of these two diseases. METHODS: We examined 112 patients with PD (53 males and 59 females; age: 77.4±3.6 years) and 28 age-matched patients with DLB (15 males and 13 females; age: 77.1±6.7 years) using brain magnetic resonance imaging (MRI) and 99 mTc-ECD SPECT subtraction imaging. RESULTS: The total number of CMBs was higher in patients with DLB (41.2%) than in those with PD (11.5%), and the prevalence was significantly higher in the former (0.7±1.1) than the latter (0.2±0.5, p < 0.05). The odds ratio was 5.4 (95% confidence interval [CI]: 1.7-17.4). Furthermore, CMBs were commonly located in the basal ganglia of patients with PD (6 out of 87 patients) but in the occipital lobe of patients with DLB (8 out of 17 patients). 99 mTc-ECD SPECT subtraction imaging indicated lower cerebral blood flow in the posterior cingulate gyrus among the patients with CMB-positive DLB than among those with CMB-positive PD; additionally, the cerebral blood flow was lower in the bilateral basal ganglia and midbrain among patients with CMB-positive DLB compared to those with CMB-negative DLB. CONCLUSION: A reduction in occipital glucose metabolism may be related to CMBs in the occipital lobe of patients with DLB.