<b><i>Background:</i></b> The stability of Alzheimer’s disease (AD) biomarkers in plasma, measured by immunomagnetic reduction (IMR) after long-term storage at –80°C, has not been established before. <b><i>Method:</i></b> Ninety-nine human plasma samples from 53 normal controls (NCs), 5 patients with amnestic mild cognitive impairment (aMCI), and 41 AD patients were collected. Each plasma sample was aliquoted and stored as single-use aliquots at –80°C. The baseline measurements for Aβ_1–40, Aβ_1–42, and total Tau protein (T-Tau) concentrations for each sample were done within 3 months of blood draw by IMR. They are referred to as baseline concentrations. A separate aliquot from each sample was assayed with IMR to assess the stability of the measured analytes during storage at –80°C between 1.1 and 5.4 years. This is referred to as a repeated result. <b><i>Results:</i></b> IMR shows that plasma levels of Aβ_1–40 and Aβ_1–42 exhibit stability over 5-year storage at –80°C and that plasma levels of T-Tau are less stable (approximately 1.5 years). <b><i>Conclusion:</i></b> Although the measured concentrations of T-Tau in human plasma may alter during storage, the diagnostic utility of the results are only slightly affected when the product of Aβ_1–42 and T-Tau concentrations are used. The results show that the overall agreement between baseline and repeated measurements in the ability of discriminating NCs from aMCI/AD patients is higher than 80%.