Pneumatosis cystoides intestinalis (PCI), characterized by presence of intramural gas cyst in the intestinal wall is associated with various medical condition. Polymyosistis, however, is rarely associated with PCI. Few cases are reported in the world, and none has not been reported previously in Korea. A 67-year-old woman with polymyositis developed mild abdominal pain and abdominal distension during treatment with steroid and azathioprine. Radiographic findings including CT scan showed intraperitoneal free gas and intramural air, compatible with PCI. The patient's symptom and clinical findings improved after the treatment with antibiotics and high-dose oxygen therapy.