Mild cognitive impairment (MCI), a syndrome characteristic of the transition phase between normal cognitive function and dementia, has been shown to carry the risk of progression to dementia. Dysregulation of blood pressure (BP) is thought to be an indicator of cerebrovascular damage, including cognitive impairment. Here, we investigated the possible association of circadian BP variation with MCI in community-dwelling persons exhibiting no definitive dementia. Our study enrolled 144 persons (68 ± 7 years). Nocturnal BP profile was defined as dipper, with a 10-19% drop in nocturnal systolic BP; extreme dipper, X20% drop; non-dipper, 0-10% drop; and riser, any increase in nocturnal BP. MCI was assessed using the MCI screen, a cross-validated, staff-administered battery of tests. Subjects with MCI (n¼38) were significantly older (74±6, 67±6 years, Po0.001) and had higher frequency of apolipoprotein E e4 allele (36.8, 18.9%, P¼0.018). Although the ambulatory measured BP and the percent changes in nocturnal systolic BP (À10 ± 12% and À12 ± 8%, respectively; P¼0.291) did not differ between MCI subjects and normal controls, frequency of MCI was significantly higher in the extreme dippers (32.0%), non-dippers (30.0%) and risers (50.0%) than in dippers (13.2%, P¼0.018). Multiple logistic regression analysis identified a blunted nocturnal BP decline, non-dipping or increase in nocturnal BP and extreme drop in BP as potent determinants of MCI (odds ratio 3.062, P¼0.039), after adjustment for possible confounding factors, including apolipoprotein E e4 genotype. Abnormal nocturnal BP profile was found to be a strong indicator of MCI in otherwise apparently healthy community-dwelling elderly persons. Keywords: ambulatory blood pressure monitoring; mild cognitive impairment; nocturnal blood pressure INTRODUCTION Mild cognitive impairment (MCI) is a syndrome defined as cognitive decline greater than expected for an individual's age and education level that does not notably interfere with activities of daily life. 1 MCI is classified into four subtypes: amnestic MCI involving only memory impairment (single domain) or memory impairment along with deficits in another cognitive domain such as language function, visuospatial skills, or executive function (multiple domain) and non-amnestic MCI involving discrete cognitive impairments in single or multiple domains other than memory function. 2 MCI prevalence has been estimated to be between 6 and 10% based on community assessments, 3 a frequency about four times that of dementia. MCI also exhibits Alzheimer-type dementia-like neuropathology on autopsy, and at least half of those diagnosed with MCI, in particular amnestic MCI subtype, eventually progress into Alzheimer-type dementia (AD) with a smaller percentage progression to other forms of dementia,