This article highlights the latest findings regarding estrogen replacement therapy in the treatment and prevention of Alzheimer's disease (AD) and mild cognitive impairment in women. Despite considerable evidence from observational studies, recent randomized clinical trials of conjugated equine estrogens, alone and in combination with progestin, have shown no benefit for either the treatment of established AD or for the short-term prevention of AD, mild cognitive impairment, or cognitive decline. Based on the evidence, there is no role at present for estrogen replacement therapy in the treatment or prevention of AD or cognitive decline, despite intriguing results from the laboratory and from observational studies. However, numerous questions remain about the biologic effects of estrogens on brain structure and function. Additional basic and clinical investigations are necessary to examine different forms and dosages of estrogens, other populations, and the relevance of timing and duration of exposure.