Observations on two patients from unrelated families are reported. Both patients were 32 years old and suffered from a congenital affection of the skeletal muscles. In one (case 2) congenital paresis of the dilator muscle of pupil and cardiomyopathy were also present. In both cases muscle biopsy showed pathologic variations of muscle fiber caliber, and in case 1 atrophic fibers were grouped in some places. Inclusions consisting of dense granular and fine filamentous components were found in skeletal muscle fibers, in about a third of the fibers in case 1 and in about a tenth of the fibers in case 2. The shape and submicroscopic structure of these inclusions were consistent with so-called cytoplasmic bodies. Surrounding or apart from the inclusions, the streaming of Z disks or their disintegration were observed. In addition, pure filamentous bodies occurred in the subsarcolemmic position in both cases. Although cytoplasmic bodies are a nonspecific type of inclusion, they were the outstanding structural anomaly in both cases. A basic defect of myofilament biosynthesis is considered to be the cause of the disorder. It is presumably evoked by a neurogenic factor affecting the trophic influence of the neuron on the muscle cell. The course of the disease may (case 1) but need not (case 2) terminate in severe impairment of skeletal muscle function.