The demonstration that myostatin may negatively regulate muscle mass in adult individuals has raised the possibility of targeting the myostatin pathway in order to increase muscle growth in a variety of muscle degenerative and wasting conditions. In this regard, blockade of endogenous myostatin results in anatomic, biochemical, and physiologic improvement in the dystrophic phenotype in the mdx mouse. Moreover, myostatin messenger ribonucleic acid levels are decreased in the regenerated muscle of these mice, suggesting that myostatin may also be involved in the pathogenesis of the disease. To gain further insight into the possible role of myostatin in muscle degenerative diseases, the present work investigates the expression of muscle myostatin in children with muscular dystrophies and mitochondrial encephalomyopathies. No differences in the pattern of myostatin expression were evident in any case, even in those patients with prominent muscular atrophy. These findings suggest that muscle loss that can be observed in muscle degenerative diseases does not depend on changes in myostatin expression.