In this systematic evidence review, we evaluate data on the use of hormone replacement therapy (HRT) to prevent cognitive decline and dementia in healthy postmenopausal women. Specifically, we reviewed the literature reporting effects of HRT on cognitive function in women without dementia. In addition, we conducted a review and meta-analysis of studies of HRT and dementia, focusing on Alzheimer disease. Results of this review have been recently published. This report is part of a larger project on the risks and benefits of HRT prepared for the U.S. Preventive Services Task Force to assist them in making recommendations. Between 3 and 8 million people in the United States have dementia. The most common type of dementia is Alzheimer disease, which affects 3 to 4 million people and is the fourth leading cause of death in the United States. The incidence of dementia is 1 percent per year in older individuals, although in the most elderly populations this rate may be as high as 2 to 3 percent. One community-based prevalence study in east Boston estimated that almost 50 percent of those aged 85 and over suffered from dementia. Most studies report that after accounting for differences in life expectancy, women have a 1.4 to 3-fold higher risk of Alzheimer disease than men. The life expectancy of demented patients is greatly reduced. Those with early-onset (before age 60) Alzheimer disease have a median survival of 6.7 to 8.1 years, while survival in those with late-onset disease is 4.8 to 6.0 years. In 1991, the annual cost of Alzheimer disease was estimated to be $67.3 billion. Given the expected growth of the elderly population, this financial cost, as well as the emotional and physical costs of caring for demented patients, will continue to increase. Observational data suggest a possible but inconsistent relationship between endogenous estrogen exposure and cognition. Women in the high estrogen phase of the menstrual cycle have been shown to perform better on tests of motor skills compared to when they are in the low estrogen phase of the cycle. Bone mineral density, hypothesized to be a marker of cumulative estrogen exposure, has been correlated with risk of cognitive deterioration. Because of the association between endogenous estrogen exposure and cognition, it can be hypothesized that hormone replacement therapy (HRT) after menopause may prevent cognitive decline and the development of dementia.