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We tested whether blood–brain barrier dysfunction in subcortical white matter is associated with white matter abnormalities or risk of clinical dementia in older people (n=126; mean age 86.4, SD: 7.7 years) in the MRC CFAS (Medical Research Council Cognitive Function and Ageing Study).Using digital pathology, we quantified blood–brain barrier dysfunction (defined by immunohistochemical labeling for the plasma marker fibrinogen). This was assessed within subcortical white matter tissue samples harvested from postmortem T2 magnetic resonance imaging (MRI)–detected white matter hyperintensities, from normal-appearing white matter (distant from coexistent MRI-defined hyperintensities), and from equivalent areas in MRI normal brains. Histopathologic lesions were defined using a marker for phagocytic microglia (CD68, clone PGM1).Extent of fibrinogen labeling was not significantly associated with white matter abnormalities defined either by MRI (odds ratio, 0.90; 95% confidence interval, 0.79–1.03; P=0.130) or by histopathology (odds ratio, 0.93; 95% confidence interval, 0.77–1.12; P=0.452). Among participants with normal MRI (no detectable white matter hyperintensities), increased fibrinogen was significantly related to decreased risk of clinical dementia (odds ratio, 0.74; 95% confidence interval, 0.58–0.94; P=0.013). Among participants with histological lesions, increased fibrinogen was related to increased risk of dementia (odds ratio, 2.26; 95% confidence interval, 1.25–4.08; P=0.007).Our data suggest that some degree of blood–brain barrier dysfunction is common in older people and that this may be related to clinical dementia risk, additional to standard MRI biomarkers.