Dementia with Lewy bodies (DLB) and Parkinson disease with dementia (PDD) have been recognized as being on the same spectrum because DLB and PDD exhibit Alzheimer disease pathology characterized by various degrees of amyloid–beta and tau in a background of Lewy bodies containing alpha–synuclein. Although these disorders are similar in neuroimaging, patients with DLB tend to show more amyloid–beta deposition, tau deposition, decreased glucose metabolism in the anterior cingulate gyrus, and thalamic atrophy than those with PDD. Furthermore, asymmetric reduction of dopamine transporter in the striatum may be stronger in patients with PDD than those with DLB, and DAT reduction is essential for diagnosis of PD, while some patients with DLB do not show DAT reduction. In terms of amyloid–beta and tau deposition, both diseases are considered to be on the same spectrum, but DLB clearly differs from PDD in that it lacks DAT reduction in some cases. There are few reports of reduced glucose metabolism extending to the frontal lobe, of thalamic atrophy and patients with normal striatal DAT uptake in DLB, and additional studies are warranted.