We have observed great alterations of the arrector pili muscle in a case of dermatomyositis. These alterations were as follows: diminution and then loss of the staining properties of the smooth muscle fibres, perinuclear vacuolization, great heterogeneity in the degeneration of fibres, elective involvement of the cyto-fibrillary system with integrity of the other constituents of the arrector muscle and the surrounding dermis. This smooth myositis is nonspecific and similar to the one observed in other internal diseases (scleroderma, chronic alcoholism).