The Women's Health Initiative (WHI) represents one of the largest studies ever conducted on women's health. WHI is comprised of a group of studies imbedded into a large cohort of over 120,000 postmenopausal women, selected from over 40 clinical centers in the United States, who are between the ages of 50 and 79 at study baseline. In WHI there are three randomized clinical trials (lowfat diet vs. usual diet; hormone therapy vs. placebo; and, calcium/vitamin D vs. placebo) and an observational study. Women are currently being enrolled into the studies and they will be followed from nine to twelve years. Primary outcomes include: cardiovascular diseases, breast and colorectal cancers, and osteoporosis.The WHI-Memory Study (WHIMS) is an ancillary study to the HRT clinical trial of WHI. Women 65 years of age and older who are participating in the HRT are eligible for recruitment into WHIMS at the time of their randomization into HRT. It is hypothesized that the development and progression of dementia will be delayed and slowed, respectively, in women who are on active HRT (ERT and PERT) as opposed to placebo. Women will be tested at baseline and annually thereafter for up to six years in order to identify dementia cases. Classification of dementia will be based on laboratory data (blood work and CT) obtained from all probable cases. It is anticipated that approximately 8,000 women will participate in WHIMS. The study is designed to provide greater than 80% statistical power to detect a 40% treatment effect on the incidence of all-cause dementia. WHIMS represents an ambitious effort to experimentally assess the efficacy of HRT in preventing and slowing the progression of dementia in post-menopausal women. Furthermore, by taking advantage of an ongoing study with an age-appropriate study cohort, trained and experienced personnel, and a strong system of quality control, WHIMS will be conducted in a relatively cost-effective manner.