Amnestic mild cognitive impairment (MCI) appears to be a very early stage of Alzheimer's disease (AD). The amyloid-beta peptide (Abeta) is believed to be a possible substrate for AD, but little is currently known about Abeta alterations in MCI and how these changes compare to later stages of disease. In the present study Abeta was differentially extracted from the brains of age-matched control, MCI, and AD cases and compared with plaque counts. For comparison, APPxPS-1 knock-in mice were processed in parallel. We observed that Abeta42 was significantly elevated in MCI subjects, even though there was no significant alteration in the total amount of Abeta. Relative Abeta solubility within the different extractable pools was identical between AD and MCI subjects, with both significantly altered relative to controls. Temporal analysis of Abeta levels and solubility in a knock-in mouse model of Abeta pathogenesis recapitulated many of the salient features observed in AD. Characterization of the SDS fraction showed some similarities between aged knock-in mice and AD subjects. These data suggest that distinct changes in Abeta occur throughout the progression of AD, and that elevations in Abeta42 occur at an early, clinically defined stage.