A patient with spontaneous acute subcutaneous emphysema and pneumomediastinum in whom peritracheal and subcarinal grossly enlarged mediastinal nodes and right main bronchus stenosis were shown by computed tomography is reported. Flexible bronchoscopy showed extrinsic compression and an endoluminal mass that occluded 70% of right main bronchus lumen. Biopsy disclosed epidermoid carcinoma. Pneumodiastinum resolved completely in a week. The spontaneous resolution of pneumomediastinum and the absence of later mediastinal infections virtually exclude main bronchus perforation. Localized airway obstruction, resulting in a ball-valve effect and distal overdistention of lung tissue, can burst alveoli and lead to disruption of alveolar walls and entry of air into the bronchovascular sheath. Common coughing in the presence of endobronchial significant obstruction may produce the required momentary discrepancy between alveolar pressure and that of the surrounding tissues. To our knowledge, only one case of pneumomediastinum in association with a lung carcinoma has been previously reported.