Objectives: To investigate dynamic changes in human plasma β-amyloid (Aβ) concentrations, evaluate the effects of aging and amyloidosis on these dynamics, and determine their correlation with cerebrospinal fluid (CSF) Aβ concentrations. Design: A repeated plasma and CSF sampling study. Setting: The Washington University School of Medicine in St Louis, Missouri. Participants: Older adults with amyloid deposition (Amyloid+), age-matched controls without amyloid deposition (Amyloid-), and younger normal controls (YNCs) were enrolled for the study. Main Outcome Measures: Hourly measurements of plasma Aβ were compared between groups by age and amyloidosis. Plasma Aβ and CSF Aβ concentrations were compared for correlation, linear increase, and circadian patterns. Results: Circadian patterns were observed in plasma Aβ, with diminished amplitudes with aging. Linear increase of Aβ was only observed for CSF Aβ in the YNC and Amyloid- groups, but not in the Amyloid+ group. No linear increase was observed for plasma Aβ. No significant correlations were found between plasma and CSF Aβ concentrations. Conclusions: Plasma Aβ, like CSF, demonstrates a circadian pattern that is reduced in amplitude with increasing age but is unaffected by amyloid deposition. However, we found no evidence that plasma and CSF Aβ concentrations were related on an hourly or individual basis.