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, >or=20% 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 (n=38) were significantly older (74+/-6, 67+/-6 years, P<0.001) and had higher frequency of apolipoprotein E varepsilon4 allele (36.8, 18.9%, P=0.018). Although the ambulatory measured BP and the percent changes in nocturnal systolic BP (-10+/-12% and -12+/-8%, respectively; P=0.291) 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%, P=0.018). 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, P=0.039), after adjustment for possible confounding factors, including apolipoprotein E varepsilon4 genotype. Abnormal nocturnal BP profile was found to be a strong indicator of MCI in otherwise apparently healthy community-dwelling elderly persons.