Three unrelated young children are reported to have suffered since birth from muscle hypotonia and two of them from fatal respiratory insufficiency. Muscle tissues were found to contain large masses of thin myofilaments, immunologically identified as containing actin, but without further morphological features. These masses of thin filaments were found in different muscles at different occasions in the three children, suggesting a disease-specific morphological and possibly nosological feature all of them justifying classification as congenital myopathy with excess of actin or actin myopathy. The lesions were dissimilar to hyaline bodies in that the latter consist of granular material which is faintly positive for ATPase activity whereas the masses of thin filaments are devoid of ATPase activity. Two of our three patients also had intranuclear rods with virtually no sarcoplasmic rods suggesting the term of this congenital myopathy as actin myopathy with intranuclear rods.