A histochemical study of biopsied muscle specimens from patients with the 3 forms of nemaline myopathy (i.e., severe neonatal, moderate congenital, and adult-onset), classified on the basis of clinical symptoms, was conducted. A close relationship could not be found between the number of rods and the severity of weakness in any form. Type 1 fiber atrophy and predominance or type 2B fiber deficiency was the usual finding in all forms. In the moderate congenital form, type 1 fiber atrophy and predominance became more apparent in patients with a protracted course, suggesting that the histochemical abnormalities are progressive. The abnormal fiber type distribution is assumed to be related to the pathogenetic mechanism of nemaline myopathy in all forms. Because acid phosphatase activity was increased in muscle fibers of patients with rapid progression, an autodegenerative process inducing lysosomal enzyme activation may be responsible for the acute clinical progression and muscle fiber loss in this disorder.