Subcutaneous emphysema is well known, but diffusion of gas into the mediastinum is not so common, particularly if it is caused by treatment of a root canal without apparent osseous fenestration or mucoperiostal lesions. We report a case of iatrogenic pneumomediastinum and facial emphysema after endodontic treatment. The diagnosis was confirmed by computed tomography, and the patient recovered after treatment with intravenous antibiotics and analgesia.