Recent reports suggest that hormone therapy may be associated with a reduced risk for Alzheimer's disease and may offer some protection against age-associated declines in specific cognitive functions. The majority of these reports are based on observational studies, which are confounded by the "healthy user" bias--the tendency for women receiving hormone therapy to be younger, better educated, and have fewer medical problems. In one attempt to address these limitations, we conducted a series of studies examining effects of hormone therapy on cognitive and brain functioning in nondemented postmenopausal women in the Baltimore Longitudinal Study of Aging (BLSA). In this sample, women receiving hormone therapy and women who never received hormone therapy were comparable with respect to educational attainment, general medical health, and performance on a test of verbal knowledge. Despite these similarities, women receiving hormone therapy performed better on tests of verbal and visual memory compared to never-treated women. The two groups also differed in the patterns of regional brain activation evoked during performance of delayed verbal and figural memory tasks. Furthermore, longitudinal comparisons revealed greater relative blood flow increases over two years in women receiving hormone therapy for the hippocampus and other mesial temporal lobe structures that subserve memory. These observational findings from our studies in the BLSA have led to the development of a large-scale randomized clinical trial of hormone therapy and cognitive aging, the ancillary Women's Health Initiative Study of Cognitive Aging (WHISCA), and have important implications for studies of the effects of SERM's on cognitive and brain functioning.