Despite clinical and neuropsychological similarities between Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB), recent studies have demonstrated that structural and pathological changes are more severe in DLB than in PDD. We used voxel-based morphometry using a 3-T MRI scanner to compare gray and white matter densities in 20 patients with probable PDD and 18 patients with probable DLB, who had similar overall severity of dementia and similar demographic characteristics. The gray matter density was significantly decreased in the left occipital, parietal, and striatal areas in patients with DLB compared with patients with PDD. The white matter density was significantly decreased in bilateral occipital and left occipito-parietal areas in patients with DLB compared with those with PDD. The degree of white and gray matter atrophy was similar in patients with DLB; in contrast, there was markedly less atrophy in the white matter than in the gray matter in patients with PDD. On analyzing the change of WM density relative to that of GM density in patients with DLB compared to those with PDD, the area of WM atrophy in the occipital areas was more extensive than that of GM atrophy. Our data demonstrate that atrophy of both gray and white matter was more severe in patients with DLB and that white matter atrophy relative to gray matter atrophy was less severe in patients with PDD. These data may reflect a difference in the underlying nature of PDD and DLB.