Good evidence suggests that estrogen affects basic neural processes and cognitive function in animals, 1,2 but the influence of estrogen on cognitive function in humans, especially postmenopausal women, has been much more difficult to establish. Results to date from observational studies and clinical trials are far from consistent. 3,4 Such inconsistency may reflect differences in the ages of the women studied. For example, midlife women tend to show a positive effect of estrogen on cognitive function. 5–7 Studies in older populations have more varied results, with some indicating a positive influence of estrogen on cognitive function, 8–13 others failing to show an effect, 14–17 and still others falling somewhere in between. 18 In animals, the effects of estrogen on cognitive function are observed most strongly when the agent is first used.