The muscles of the body sometimes become inflamed, resulting in symmetrical weakness of the legs and arms. (When the cause of inflammation is not known, it is referred to as idiopathic.) This weakness is unrelated to exercise and may be accompanied by muscle aching or tenderness. The two main types of idiopathic muscle inflammation are known as polymyositis and dermatomyositis. Skin lesions or rashes often accompany dermatomyositis. Since the early 1900s, researchers and clinicians have noted that the risk for cancer seems to be increased in people with idiopathic inflammatory muscle disease compared with the general population. Just how great the risk for cancer is in people with inflammatory muscle disease has not been clear, however.