Polymyositis (PM) is an idiopathic inflammatory myopathy that affects striated muscles. Dermatomyositis (DM) is an idiopathic inflammatory myopathy with presence of skin symptoms. Both are characterized by acute or subacute onset, symmetrical proximal muscle weakness, the presence of mononuclear cell infiltrates of the muscles and increased activity of muscle enzymes. The treatment still remains glucocorticoids and disease-modifying drugs. Symptoms of PM/DM can be a signal of developing cancer. Known risk factors for cancer in patients with PM/DM are older age, male gender, dysphagia, skin necrosis, cutaneous vasculitis, rapid onset of the disease, elevated creatinine kinase (CK) and C reactive protein (CRP), and an increase in the erythrocyte sedimentation rate (ESR). Recently three new myositis-specific autoantibodies (MSA) predicting the risk of cancer have been discovered: melanoma differentiation-associated protein 5 (anti-MDA-5), transcription intermediary factor 1γ (TIF-1γ), and nuclear matrix protein NXP-2.