Background and Purpose Vascular dementia is the second most common form of dementia but reliable evidence on age-specific associations between blood pressure and risk of vascular dementia is limited and some studies have reported negative associations at older ages. Methods In a cohort of 4.28 million individuals, free of known vascular disease and dementia and identified from linked electronic primary care health records in the UK (Clinical Practice Research Datalink), we related blood pressure to time to physiciandiagnosed vascular dementia. We further determined associations between blood pressure and dementia in a prospective population-based cohort of incident TIA and stroke (Oxford Vascular Study). Results Over a median follow-up of 7.0 years, 11 114 initial presentations of vascular dementia were observed in the primary care cohort after exclusion of the first four years of follow-up. The association between usual systolic blood pressure (SBP) andrisk of vascular dementia decreased with age (HR per 20 mm Hg higher SBP = 1.62, 95% CI 1.13-2.35 at 30-50 years; 1.26, 1.18-1.35 at 51-70 years; 0.97, 0.92-1.03 at 71-90 years; p trend = 0.006). Usual SBP remained predictive of vascular dementia after accounting for effect mediation by stroke and TIA. In the population-based cohort, prior SBP was predictive of 5-year risk of dementia with no evidence of negative association at older ages. Conclusions Blood pressure is positively associated with risk of vascular dementia, irrespective of preceding TIA or stroke. Previous reports of inverse associations in old age could not be confirmed.