Sporadic inclusion-body myositis (s-IBM), the most common muscle disease of older persons, is of unknown cause and there is no successful treatment. We summarize our most recent findings, which provide a better understanding of the steps in the pathogenetic cascade. We suggest that s-IBM is primarily a myodegenerative disease. Intriguing are the phenotypic similarities between s-IBM muscle fibers and the brains of Alzheimer disease, the most common neurodegenerative disease of older persons. In s-IBM, abnormal accumulation of the amyloid-beta (Abeta) precursor protein and its proteolytic fragment, Abeta, associated with the aging intracellular milieu of the muscle fiber, appear to be key upstream pathogenic events. We propose that the identified abnormal accumulation, misfolding, and aggregation of proteins, perhaps provoked by the aging milieu and aggravated by the oxidative stress, lead to the s-IBM-specific vacuolar degeneration and atrophy of muscle fibers.