A case of pneumomediastinum and subcutaneous cervical emphysema during the extraction of third molars under general anesthesia has been presented. The most likely cause was pulmonary barotrauma secondary to a faulty expiratory valve. This emphasizes the need for the proper preoperative check of all anesthetic equipment. Although pneumomediastinum and subcutaneous cervical emphysema are usually self-limiting conditions with rapid recovery, the patient must undergo close observation for the possible development of serious complications.