1. The authors report two siblings with dementia with Lewy bodies (DLB). Both the older brother and the younger sister underwent positron emission tomography (PET) studies with [18F]-2-fluoro-deoxy-D-glucose (FDG) during life. The FDG-PET study demonstrated unique and pronounced metabolic impairment in the occipital cortex in both patients. The clinical diagnosis of DLB in the sister was confirmed by autopsy. 2. FDG-PET images from 11 patients with Alzheimer's disease (AD), 7 patients with DLB and 10 age-matched normal subjects were obtained and analyzed by the statistical parametric mapping (SPM) method. The SPM demonstrated a widespread metabolic reduction in the DLB group. The reduction was particularly pronounced in the visual association cortex in the DLB group compared to the AD group irrespective of clinical severity of the disease. 3. These findings suggest that functional neuroimaging techniques, including FDG-PET, will provide a valuable diagnostic aid to differentiate DLB from AD, and this will help detect DLB patients in the early stage of the disease.