Apparently normal dendrites from the frontal cortex of 7 patients with Alzheimer's disease showed mitochondria with increased density matrices and paracrystalline inclusions in the intercristal space. Most of the inclusions were formed by parallel laminae, while others presented a polyhedric arrangement. Deposits of electron-dense flocculent material were also seen. These abnormal mitochondria appeared in areas of the cortex where dendritic degeneration was more intense. These results, together with identical observations made in the cortex of patients with subacute sclerosing panencephalitis, where dendritic degeneration is also known to occur, led us to advance that mitochondrial abnormalities might precede dendritic degeneration.