Characterized by inflammation of skeletal muscle with weakness. Sometimes it is associated with a skin rash (hence, dermatomyositis). It is seen in ages 40-60, but also in ages 5-15, mostly in women. Some of these patients have Jo-1 antibody. Inflammation in polymyositis is mainly mediated by cytotoxic CD8 cells. In dermatomyositis, mainly antigen-antibody complexes produce a vasculitis in muscle and skin. Some adults (10-20%) develop cancer.