Previous studies have reported a higher prevalence of dementia in persons with low blood pressure. To examine whether low blood pressure is prospectively associated with the occurrence of Alzheimer disease and dementia in elderly people. A community-based, dementia-free cohort (n = 1270) aged 75 to 101 years was longitudinally examined twice within 6 years to detect incident dementia using the Diagnostic and Statistical Manual of Mental Disorders, Revised Third Edition criteria. Cox proportional hazards models were used to analyze blood pressure in association with dementia after adjustment for several potential confounders. During the 6-year period, 339 subjects were diagnosed with dementia, including 256 persons with Alzheimer disease. Subjects with very high systolic pressure (>180 vs 141-180 mm Hg) had an adjusted relative risk of 1.5 (95% confidence interval [CI], 1.0-2.3; P =.07) for Alzheimer disease, and 1.6 (95% CI, 1.1-2.2) for dementia. Low systolic pressure (90 mm Hg) was not associated with dementia incidence, whereas extremely low diastolic pressure (