We identified axonal defects in mouse models of Alzheimer's disease that preceded known disease-related pathology by more than a year; we observed similar axonal defects in the early stages of Alzheimer's disease in humans. Axonal defects consisted of swellings that accumulated abnormal amounts of microtubule-associated and molecular motor proteins, organelles, and vesicles. Impairing axonal transport by reducing the dosage of a kinesin molecular motor protein enhanced the frequency of axonal defects and increased amyloid-β peptide levels and amyloid deposition. Reductions in microtubule-dependent transport may stimulate proteolytic processing of β-amyloid precursor protein, resulting in the development of senile plaques and Alzheimer's disease.