Duchenne muscular dystrophy is a lethal genetic disease of childhood caused by primary abnormalities in the gene coding for the membrane cytoskeletal protein dystrophin. The mdx mouse is an established animal model of various aspects of X-linked muscular dystrophy and is widely used for studying fundamental mechanisms of dystrophinopathy and testing novel therapeutic approaches to treat one of the most frequent gender-specific diseases in humans. In order to determine global changes in the muscle proteome with the progressive deterioration of mdx tissue with age, we have characterized diaphragm muscle from mdx mice at three ages (8-weeks, 12-months and 22-months) using mass spectrometry-based proteomics. Altered expression levels in diaphragm of 8-week vs. 22-month mice were shown to occur in 11 muscle-associated proteins. Aging in the mdx diaphragm seems to be associated with a drastic increase in the extracellular matrix proteins, collagen and dermatopontin, the molecular chaperone αB-crystallin, and the intermediate filament protein vimentin, suggesting increased accumulation of connective tissue, an enhanced cellular stress response and compensatory stabilization of the weakened membrane cytoskeleton. These proteomic findings establish the aged mdx diaphragm as an excellent model system for studying secondary effects of dystrophin deficiency in skeletal muscle tissue.