Two patients, a brother and sister, both suffering from congenital generalized muscle weakness with a progressive course are reported. Muscle biopsy revealed areas with loss of cross striations in the muscle fibers, electronmicroscopically presenting complete disorganization of the myofibrils with streaming of the Z discs and absence of mitochondria. Vesicular nuclei with prominent nucleoli were present in these areas. There was a type I fiber predominance in both cases. The mean diameter of the type I muscle fibers in one of the cases was too small. Motor endplate alterations in this patient gave no evidence of denervation but were suggestive of a delayed development of motor nerves.