Abstract: Objective To analyze the clinical characteristics of pneumomediastinum in patients with dermatomyositis and polymyositis and to study its pathogenesis and prognosis. Methods The clinical records of 96 patients with PM/DM were reviewed, focusing mainly on the presence of pneumomediastinum. Five patients with pneumomediastinum are described in detail. Case reports of pneumomediastinum in PM/DM in English publications are reviewed. Results Five DM cases complicated by pneumomediastinum all had lung infections. 29 cases (including our five cases) of DM/PM with pneumomediastinum have taken methylprednisolone, four cases alive, and six died. Ten cases have taken CsA，eight cases alive and two died. Conclusions The infections was strongly suspected as being responsible for the pneumomediastinum. methylprednisolone has poor effect. CsA can be an effective therapeutic agent in PM/DM.

Key words: Polymyositis, Dermatomyositis, Pneumomediastinum