Purpose : To tabulate underlying disease and to assess the clinical significance of CT-diagnosed spontaneouspneumonediastinum. Materials and Methods : We retrospectively reviewed CT scans and medical records of 11consecutive patients with spontaneous pneumomediastinum, and analyzed their clinical history and course, and infive cases, pulmonary function. CT scans of 126 patients with idiopathic pulmonary fibrosis (IPF) collected whilethe 11 consecutive patients were being treated were analyzed for the prevalence of pneumomediastinum. We analyzedCT findings with respect to the amount and distribution of air in the mediastinum, and the presence or absence ofair outside the mediastinum. Results : In the 11 patients, underlying diseases were IPF (n=4), bronchiolitisobliterans organizing pneumonia (BOOP)(n=2), bronchiectasis (n=2), tuberculous tracheal stenosis (n=1), andpulmonary tuberculosis with bullous emphysema (n=1); there was one without associated disease. Of the 126 patientswith IPF, four (3.2%) showed spontaneous pneumomediastinum. All ten with underlying diseases had severe dyspnea.In five patients, a pulmonary function test showed marked impairment. Two of four patients with IPF and both withBOOP died within 2 months of CT scanning, whereas the remaining six showed clinical improvement. The detectionrate of pneumomediastinum on plain chest radiograph was 82% (9/11). CT showed that mediastinal air was mostfrequently found in the retrosternal space. There were four cases of pneumothorax and two of subcutaneousemphysema. Conclusion : Spontaneous pneumomediastinum might be associated with idiopathic pulmonary fibrosis andmight be a poor prognostic factor in patients with IPF or BOOP.