The role of HRT and/or oestrogens in the aetiology, progression and prevention of neurodegenerative disorders is unclear. Various studies have been conducted in cells, animals and humans to better define these relationships. The following is a presentation of data on the role of oestrogens in the two most common dementia syndromes Alzheimer's Disease and vascular dementia. This review examines whether there is support for the hypothesis that the use of oestrogens reduces the risk for dementia and cognitive decline in a variety of experimental models. A literature search was performed using the following key words: oestrogen and dementia, oestrogen and Alzheimer, oestrogen and cognition, oestrogen and learning, and oestrogen and memory. The reference lists for the articles identified using these search strategies were examined. The strongest evidence for a beneficial effect comes from cell and animal studies. These suggest that treatment with oestrogens may influence the pathogenetic processes of AD, mainly by affecting the beta-amyloid metabolism, by promoting the activity of the cholinergic system, by reducing oxidative stress or cardiovascular risk. Observational studies give some support to the hypothesis that HRT may be protective for cognitive decline and dementia in the elderly, but the inherited methodological problems in these studies preclude any conclusions. The treatment trials published thus far have methodological problems that prohibit definite conclusions on the relationship between HRT and cognitive decline and dementia.