Progressive muscular dystrophy is characterized by muscle fiber necrosis, regeneration, and endomysial fibrosis. Although absence of dystrophin has been known as the cause of muscle fiber degeneration, pathogenesis of interstitial fibrosis is still unknown. Transforming growth factor-beta 1 (TGF-beta 1) induces accumulation of extracellular matrix in various diseases, such as liver cirrhosis and interstitial pneumonitis. To investigate its function on the pathogenesis of progressive muscular dystrophy, it was necessary to determine the degree of TGF-beta 1 expression and the site of TGF-beta 1 immunoreactivity. In Duchenne muscular dystrophy and most of Becker muscular dystrophy, high TGF-beta 1 immunoreactivity expressed on muscle fibers and extracellular space. In other myopathies with endomysial fibrosis, however, TGF-beta 1 was seldom observed. We also examined the immunoreactivity of the latent TGF-beta binding protein, which is bound to the TGF-beta precursors. In all Duchenne muscular dystrophy and half of Becker muscular dystrophy cases, high latent TGF-beta 1 binding protein immunoreactivity was seen, but in other myopathies its immunoreactivity was seldom seen on muscle fibers or extracellular space. Therefore TGF-beta 1 may play an important role in synthesis and accumulation of extracellular matrix in progressive muscular dystrophy.