A 65-year-old woman showed the clinical picture of a myogenic myopathy, confirmed electromyographically. Detailed histopathological, histochemical and ultrastructural studies were performed on deltoid and quadriceps femoris biopsy specimens. Many muscle fibres were found to contain vacuolar spaces with lipid droplets: perivascular inflammatory changes were also observed. The affected fibres were consistently type I. Under the electron microscope the fibres contained large numbers of lipid droplets, usually next to normal mitochondria. A slight improvement occurred after prednisone therapy.