SUMMARYA 38-year-old woman has had a slowly progressive myopathy of ten years' duration, involving proximal and distal limb and axial muscles. There is no family history of neuromuscular disease, no evidence of a systemic illness, and no exposure to drugs or toxins. A recent muscle biopsy showed an abundance of vacuoles and membranous bodies containing phospholipids in many muscle fibers of both histochemical types. By electron microscopy there were accumulations of peculiar 180-Ang-ström cytoplasmic and nuclear filaments suggestive of viral nucleocapsids. The total phospholipid concentration of the muscle biopsy, as determined by thin-layer chromatography, was increased threefold. All major classes of phospholipids showed a proportionate increase. It is suggested that this case may represent a chronic progressive viral disease of skeletal muscle with a secondary disturbance of cellular phospholipid metabolism.