The aim of this study was to investigate whether amyloid deposition is associated with Alzheimer's disease (AD)-like cortical atrophy in Lewy body (LB) disease (LBD). Participants included 15 LBD with dementia patients (8 with dementia with Lewy bodies [DLB] and 7 with Parkinson's disease [PD] with dementia [PDD]), 13 AD patients, and 17 healthy controls. Age, gender, and Mini-Mental State Examination scores were matched between patient groups. All subjects underwent PET scans with [(11)C]Pittsburgh Compound B to measure brain amyloid deposition as well as three-dimensional T1-weighted MRI. Gray-matter volumes (GMVs) were estimated by voxel-based morphometry. Volumes-of-interest analyses were also performed. Forty percent of the 15 DLB/PDD patients were amyloid positive, whereas all AD patients and none of the healthy controls were amyloid positive. Amyloid-positive DLB/PDD and AD patients showed very similar patterns of cortical atrophy in the parahippocampal area and lateral temporal and parietal cortices, with 95.2% of cortical atrophy distribution being overlapped. In contrast, amyloid-negative DLB/PDD patients had no significant cortical atrophy. Compared to healthy controls, parahippocampal GMVs were reduced by 26% in both the amyloid-positive DLB/PDD and AD groups and by 10% in the amyloid-negative DLB/PDD group. The results suggest that amyloid deposition is associated with AD-like atrophy in DLB/PDD patients. Early intervention against amyloid may prevent or delay AD-like atrophy in DLB/PDD patients with amyloid deposition.