Histological and ultrastructural alterations of skeletal muscles in Duchenne muscular dystrophy (DMD) (32 cases) and polymyositis (PM) (33 cases) were compared qualitatively and quantitatively. Regeneration is more numerous in PM than DMD throughout all stages. Stromal fibrosis and hyaline fiber with delta lesion are characteristic for DMD. In the electron microscope, we observed the usual preservation of basement membrane in degenerating muscle fibers and uncomplicated muscle regeneration in PM. On the other hand, there were partial disruption of the plasma membrane lining the delta lesion and degeneration of basement membrane and probable invasion of collagen fiber into sarcoplasm in some hypercontracted fibers in DMD. Besides, there appeared atypical form of regeneration in DMD, namely myocytes arrested at the stage of mononuclear myoblast or primitive myotube with degeneration of myofibrils and cytoplasmic organelles. At the advanced stage of DMD, there was scarce activity of regeneration. Stromal fibrosis, abnormality of membrane systems, and failure of regeneration were the characteristics of DMD.