A decade ago, oestrogen-containing hormone therapy was viewed as a promising strategy for the prevention and treatment of dementia and age-related cognitive decline. However, treatment trials in women with Alzheimer's disease showed that oestrogens did not reverse cognitive impairment, and clinical trials in healthy older women indicated that oestrogens did not prevent cognitive decline. The Women's Health Initiative Memory Study trial even suggested an increased risk of dementia with treatment late in life. What happened? How are we to understand these findings? What are the implications for...