Four of seven patients with nemaline myopathy had severe, rapidly progressing symptoms. These four showed an increase in acid phosphatase activity in muscle fibers demonstrated by histochemistry and cathepsin B&L activity by biochemical measurement. On electron microscopy, nemaline bodies, occasionally disorganized myofibrils and autophagic vacuoles containing sarcoplasmic debris and glycogen particles were seen. Focal myofibrillar degeneration, through an unknown pathogenetic mechanism, induces an increase in lysosomal enzymes in the skeletal muscles which may be closely correlated with a rapid aggravation of muscle weakness in nemaline myopathy.