Two cases of elastofibroma dorsi are reported and the light and electron microscopic features analysed. The appearances suggest that the characteristic fibres and globules that stain as for elastin arise by denaturation of collagen. The fibres differ from the elastin of arterial wall, elastotic degeneration of skin, and pseudoxanthoma elasticum. The morphology and clinical behaviour indicate that elastofibroma dorsi is a reactive fibromatosis rather than a true neoplasm.