Many muscular dystrophies arise from enhanced muscle degeneration, but one muscular dystrophy subtype has now been shown to arise from defective muscle membrane repair. Mutations in the gene encoding dysferlin cause muscular dystrophy, and recent work has demonstrated a role for this protein in resealing muscle membrane tears. Thus, two broad categories of muscle membrane defects can now be defined: those with inherent instability and those with compromised repair. The latter might be relevant for muscle wasting that occurs with aging.