The medial temporal lobe is affected early on in Alzheimer's disease (AD), and the presence of medial temporal atrophy can be used in the differential diagnosis of AD. However, the hippocampus has a complex structure, and subregions of the hippocampus are differentially affected in different dementia types. We used high resolution (0.3 mm in-plane) coronal 3T MR imaging of the medial temporal lobe in 16 subjects with Alzheimer's disease (AD), 16 with dementia with Lewy bodies (DLB) and 16 similarly aged healthy subjects to investigate differences in the hippocampus subregions. On the anterior section of the hippocampus body, regions of interest were manually drawn blind to diagnosis on the CA1, CA2 & CA3/4 subregions, and the width of the subiculum and entorhinal cortex was measured. Controlling for intracranial volume, age and years of education, we found the subiculum thickness was significantly reduced in AD (2.03 ± 0.29 mm) compared to both control (2.37 ± 0.28 mm, p = 0.008) and DLB (2.35 ± 0.24 mm, p = 0.001) subjects. The area of CA1 was likewise reduced in AD compared to controls and DLB. In the hippocampus images, a hypointense line is visible between CA1 and CA3/4. This line was significantly less distinct in AD, suggesting disease related changes to this region. Future studies should investigate whether subiculum thickness or the hypointense line could be a diagnostic feature to help discriminate AD from DLB.