In patients with Alzheimer's disease, the brain interstitial space is an important place where amyloid-β oligomers and aggregates exist. Although tau aggregates are observed inside neurons, extracellular brain interstitial fluid tau has drawn attention because of increasing understanding of cell-to-cell propagation of tau aggregation. In this review, we summarize our current understanding of factors influencing brain interstitial fluid concentrations of amyloid-β and tau, mainly focusing on known epidemiological risk factors for Alzheimer's disease.