The decline in circulating oestrogen concentrations that occurs after the menopause has the potential to impact Alzheimer's disease and other forms of dementia. Relevant actions include neurotrophic and neuroprotective effects; effects on acetylcholine and other neurotransmitters; and effects on proteins implicated in Alzheimer's disease pathogenesis. Since 1990, 14 case-control and cohort studies have considered the relation between postmenopausal oestrogen therapy and Alzheimer's disease. Most, but not all, report that oestrogen therapy is associated with a reduced Alzheimer risk of approximately one-half. Almost no epidemiological data address the potential link between oestrogen and other forms of dementia. Several small interventional trials have considered whether oestrogen might improve cognitive function of women with Alzheimer's disease. Data, however, are limited, and there is no compelling evidence that the short-term use of oestrogen monotherapy has a substantial impact on dementia symptoms. In summary, the use of oestrogen to reduce Alzheimer risk is biologically credible, and the preponderance of epidemiological evidence suggests that oestrogen therapy is indeed protective. This potentially important role of oestrogens for the primary prevention of Alzheimer's disease remains to be verified through well-designed randomized controlled trials.