The reason of the disease is supposed to be an autoimmune response to muscle tissue of persons who are genetically predisposed. Possible starting factors are malignant neoplasms and virus myositis. There is a research of the detection in muscle cells of structures similar to picornaviruses; also, viruses may induce similar diseases in animals. The association of malignant neoplasms with dermatomyositis suppose that neoplasm growth can also be a trigger mechanism for the development of the disease as a result of triggering autoimmune reactions to the common tumor and muscle antigens. Deposits of IgM, IgG and the third complement component are detected in the walls of the blood vessels of skeletal muscles.