Background/Aims: Investigations of Aß oligomers in neuropathologically confirmed Alzheimer disease (AD) are still scarce. We report neurohistopathological and biochemical analyses using antibodies against tau and amyloid β pathology. Methods: Thirty elderly AD patients and 43 age-matched controls with or without deposition of amyloid plaques (AP) were analysed by immunohistochemistry. In 21 cases with available fresh tissue, Western blots were also performed. Neuropathological analysis included quantitative assessment of neurofibrillary tangles (NFT), amyloid plaques and Aß oligomer densities in mesial temporal cortex. Results: NFT, fibrillar amyloid and Aß oligomeric deposits densities were significantly higher in AD compared to controls. There was no relationship between oligomeric Aß densities and Braak NFT staging scores. Furthermore, Aß oligomer expression was closely correlated to Aß plaques in the temporal cortex. By Western blot, Aß oligomers were observed in AD patients, in plaques-free controls, in one “tangle-only AD” case, as well as in the cerebellum. A band near 55 kDa was the only Western blot signal that was significantly increased in temporal cortex of AD patients compared to controls as well as less expressed in the cerebellum. Conclusion: These results suggest that a putative dodecamer, near 55 kDa, may contribute to AD vulnerability of the temporal cortex.