In this review, we synthesize evidence on associations between estrogen—specifically age at menarche and menopause, pregnancy, menstrual cycle patterns, and exogenous estrogen use (contraception, HRT)—and known potentially modifiable risk factors for dementia, APOE ε4, as highlighted by the 2024 Lancet Commission on dementia prevention, intervention, and care.33 This evidence concerns the gene APOE and dementia.