Postmenopausal estrogen-based hormone therapy lasting longer than ten years was associated with a decreased risk of Alzheimer’s disease in a large study. The study explored the association between postmenopausal hormone replacement therapy, Alzheimer’s disease, dementia and cognition in two nation-wide case-control studies and two longitudinal cohort studies. The largest study comprised approximately 230,000 Finnish women and the follow-up time in different studies was up to 20 years.