We compared regional cerebral blood flow (CBF) patterns in patients with dementia with Lewy bodies (DLB) and Alzheimer's disease (AD) using single photon emission computed tomography (SPECT) and investigated the diagnostic utility of SPECT study in differentiating between DLB and AD. SPECT data on 20 patients with DLB and 75 patients with AD were analyzed using three-dimensional stereotactic surface projections. Regional CBF reduction was determined by quantitative analysis using stereotactic extraction estimation method. The DLB group showed a significant CBF reduction in the temporoparietal, frontal lobe and posterior cingulate, similar to the CBF pattern in the AD group, but regional CBF in the medial and lateral occipital lobes decreased significantly in patients with DLB compared with patients with AD. Receiver operating characteristic analysis revealed that regional CBF measurement of the medial occipital lobe, including the cuneus and lingual gyrus, yielded a sensitivity of 85% and a specificity of 85% in discriminating DLB from AD. Objective and quantitative CBF measurement in the medial occipital lobe may be useful in the clinical differentiation of DLB and AD.