A silent stroke is a type of stroke that does not produce any severe symptoms, but may slightly affect memory or thinking abilities. Women who take hormone replacement therapy (HRT) may have a higher risk of experiencing a silent stroke than women who do not use HRT. This study will use magnetic resonance imaging (MRI) to determine the prevalence of silent strokes and other changes in brain tissue in women who participated in the Women's Health Initiative Memory Study (WHIMS), a study in which women received HRT, either as estrogen alone or as estrogen and progesterone combined, or placebo.