Autoimmune disease of skin and muscle. Degeneration, regeneration, edema, infiltration by lymphocytes in both tissues. Inflammatory cells (predominantly B cells) around small vessels or in perimysium; absent in 25% of biopsies. Complement deposits in capillaries (vasculopathy). Muscle degeneration and regeneration most marked at periphery of muscle bundles (perifascicular atrophy).

Incidence, together with polymyositis: seven cases per year per million people.

Occurs in all decades of life; peaks before puberty and age 40. About 10% of cases starting after age 40 associated with malignant neoplasm.