Respiratory muscle involvement in nemaline myopathy. A boy who had experienced generalized muscle weakness and hypotonia since early infancy was diagnosed as having nemaline myopathy on the basis of muscle biopsy at 3 years of age. At 8 years of age, he developed severe respiratory failure and required respiratory support during sleep. Because of recurrent pneumothorax, he underwent thoracic surgery, at which time biopsy specimens were obtained from the respiratory and truncal muscles. The histologic findings of the respiratory muscles included marked variation in fiber size with a notable increase in fibrous tissue, type 2 fiber deficiency, elevated acid phosphatase activity, and a disorganized intermyofibrillar network. The findings from the truncal muscles were similar to those of the biceps brachii muscle: little variation in fiber size, numerous nemaline bodies in all fibers, and type 1 fiber predominance. The preferential damage to the respiratory muscles was probably responsible for the sudden onset of severe respiratory failure. 