A search for genetic influence in idiopathic inflammatory myopathy

33 patients with idiopathic inflammatory myopathies (polymyositis or dermatomyositis) and 45 of their first-degree relatives were investigated in a search for any influence of genetic factors in these diseases. None of the relatives had evidence of an inflammatory myopathy but 13 had some other autoimmune disease. Mean serum IgG levels were reduced and serum C3c concentration increased both in patients and relatives. Levels of IgM were reduced and C4 increased in some groups of patients. The incidence of autoantibodies was increased in the patient group, particularly in those with isolated dermatomyositis or other systemic features, but not in the relatives. HLA, blood group and other genetic markers showed no deviation from normal population frequencies. Evidence favouring a genetic influence on the etiology of idiopathic inflammatory myopathy is restricted to disturbance of levels of some Ig and complement components, and for these there may be other explanations.