INTRODUCTION: Midlife hypertension is associated with dementia risk, although uncertainties remain regarding its association with subtypes and regarding the effect of pregnancy-related hypertension on dementia risk. METHODS: In the Million Women Study, 1,363,457 women (mean age 57) were asked about current treatment for hypertension and hypertension in pregnancy and were followed for first hospital record with any mention of dementia. Cox regression yielded hazard ratios (HRs) adjusted for socioeconomic, lifestyle, and metabolic factors. RESULTS: With 84,729 dementia cases over 21 years, midlife hypertension was positively associated with dementia (HR 1.17, 95% confidence interval [CI] 1.15 to 1.19); higher for vascular dementia (VaD) (HR 1.50; 95% CI 1.45 to 1.56) than Alzheimer's disease (AD) (HR 1.01; 95% CI 0.98 to 1.04). Hypertension in pregnancy but not in midlife was only weakly associated with dementia (HR 1.04; 95% CI 1.01 to 1.06). DISCUSSION: Midlife hypertension is a strong risk factor for dementia, largely through VaD. Hypertension during pregnancy does not appear to materially affect dementia risk. HIGHLIGHTS: Midlife hypertension was associated with long-term all-cause dementia risk. Midlife hypertension was associated with VaD, not AD. Hypertension in pregnancy has little effect on dementia risk.