Muscle biopsies from 32 cases with polymyositis and dermatomyositis were examined by electron microscopy. Most of the changes of the muscle fibres were non-specific and little structural difference was seen in the various clinical types. The muscle fibres sometimes showed rough endoplasmic reticulum, annulate lamellae and prominent Golgi apparatus. Thin and thick filamentous inclusions were present both in the sarcoplasm and nuclei of some muscle fibres. Thick filamentous inclusions were seen in chronic cases and were sometimes associated with annulate lamellae. Regeneration of muscle was often conspicuous. The endothelial cells of the blood vessels were hypertrophied; some cells contained granulotubular inclusions, rod-shaped bodies (Weibel-Palade bodies) and filamentous material. The basement membrane of the vessels was often multi-layered. The cells infiltrating the interstitial tissue included macrophages, lymphocytes, transformed lymphocytes, plasma cells, monocytes, mast cells and only occasional eosinophils and basophils. Lymphocytes, macrophages and a few plasma cells were seen between the plasma and basement membranes of degenerating muscle fibres. Changes were also noted in some intramuscular nerves, motor end-plates and a muscle spindle.