Vascular lesions have been discovered by a number of authors in children's dermatomyositis, when they are not ordinary found in adults. Affecting capillaries, arterioles and veinlets, vasculitis appears as an hyperplasia of intima, then as an endothelial necrosis and formation of thrombi. Usually seen with light microscope, these lesions are sometimes only revealed by electron microscope. These vascular lesions are observed in the muscle and mainly in the perimysial interstitial tissue, in the skin, and in the various visceral localisations of the disease: pulmonary, ocular, renal, nervous and mainly digestives. The ulcerations of the digestive tube are due to little infarcts secondary to submucous arteriolar occlusions. The presence of necrotizing microangiopathy in the muscle, in the skin and in the viscera, and the observation by an author of macroglobulin, gammaglobulin and C3 deposits in the vascular walls allows to issue an aetiopathogenic hypothesis. The vascular lesions could be due, at least partly, to immune complex deposits, explained by an allergic mechanism of type III of Gell and Coombs. This process could be released by a viral factor which has to be determined. This pathogeny could bring children's dermatomyositis nearer to the other circulating immune complexe collagenosis: S. L. E., periarteritis nodosa with which many transition form exist, and could separate it to the adults dermatomyositis, which would rather be a disease due to an exacerbation of the delayed hypersensitivity type IV of Gell and Coombs.