We investigated blood-brain barrier (BBB) function in relation to Alzheimer's disease (AD) and vascular dementia (VAD) in the very elderly. Sixty-five 85-year-old persons from a population-based sample were followed for 3 years; 29 were demented at age 85 (13 with AD, 14 with VAD, and 2 with other dementias), 7 developed dementia during follow-up, and 29 remained nondemented. CSF/serum albumin ratio was used as as a measure of BBB function. Dementia was defined according to the DSM-III-R, AD according to the NINCDS-ADRDA criteria, and VAD according to the NINDS-Association Internationale pour la Recherche et l'Enseignement en Neurosciences (AIREN) criteria. Mean CSF/serum albumin ratio was higher in all dementias (8.5± 4.3; p = 0.007) and in the subtypes AD (8.9 ± 5.3; p = 0.046) and VAD (8.7 ± 3.5; p= 0.002) than in nondemented individuals (versus 6.5 ± 2.0), but it was not related to dementia severity. Nondemented women at age 85 (n = 3) who developed dementia during the follow-up had a higher CSF/serum albumin ratio than those not developing dementia (10.4 ± 2.0 versus 6.0 ± 1.9; p = 0.007). Nondemented individuals lacking the apolipoprotein E ε3 allele (n = 4) had a higher CSF/serum albumin ratio (9.3 ± 0.8 versus 6.6 ± 2.1; p = 0.029) than other individuals. A relative BBB dysfunction is associated with both AD and VAD among very elderly individuals. This finding is possibly found early in the disease before the onset of clinical dementia.