Before the use of corticosteroids, the prognosis for polymyositis/dermatomyositis (PM/DM) was extremely poor. To date, although overall prognosis appears to be better, PM and DM are still considered to be associated with increased morbidity, primarily related to severe muscle weakness and visceral involvement. Recent series underline that only 20% to 40% of treated patients will achieve PM/DM remission, whereas 60% to 80% will experience a polycyclic or chronic, continuous course of the disease. PM/DM further continues to have a great impact on life in medium- and long-term follow-up, as up to 80% of treated patients are still disabled (using Health Assessment Questionnaire scores). The overall mortality ratio in PM/DM patients also remains threefold higher compared with the general population, with cancer, lung, and cardiac complications and infections being the most common causes of deaths. Predictive factors for a poor prognosis in PM/DM patients are older age, involvement of lung and cardiac systems, dysphagia, cancer, and serum myositis-specific antibodies (including coexistent presence of anti-Ro52 and anti-Jo1 antibodies, anti-signal recognition particle antibody, anti-155/140, and anti-CADM-140 antibodies).