Background and Purpose—Low blood pressure has been related to an increased risk of dementia. We sought to verify blood pressure variations before and after a dementia diagnosis and to relate blood pressure decline to subsequent Alzheimer disease and dementia.

Methods—A community dementia-free cohort aged _75 years (n_947) underwent follow-up examinations twice over a period of 6 years to detect dementia cases (Diagnostic and Statistical Manual of Mental Disorders, 3rd edition, revised [DSM-III-R] criteria, n_304). Blood pressure variation before and after dementia diagnosis was verified with linear mixed-effects models. Using the dementia-free cohort identified at first follow-up (n_719), the association between blood pressure decline from baseline to first follow-up and subsequent risk of dementia was examined.

Results—Blood pressure markedly decreased over 3 years before dementia diagnosis and afterward, whereas no substantial decline was present 3 to 6 years before the diagnosis. However, among subjects with baseline systolic pressure _160 mm Hg, systolic pressure decline _15 mm Hg occurring 3 to 6 years before diagnosis was associated with relative risks (95% CI) of 3.1 (1.3 to 7.0) for Alzheimer disease and 3.1 (1.5 to 6.3) for dementia. There was a dose–response relationship between systolic pressure decline and dementia risk in subjects with vascular disease.

Conclusions—Blood pressure starts to decrease only 3 years before dementia diagnosis and continues to decline afterward. A greater decline in systolic pressure occurring 3 to 6 years before diagnosis is associated with an increased risk of dementia only in older people with already low blood pressure or affected by vascular disorders.