Pneumomediastinum with subcutaneous emphysema is a relatively rare complication of dental treatment. The most common dental etiology of this complication is the introduction of air via the air-turbine handpiece during surgical extraction of an impacted tooth, but this complication is very rare after nonsurgical endodontic treatment. We report herein 2 cases of massive pneumomediastinum and cervicofacial subcutaneous emphysema that developed after opening an access cavity for endodontic retreatment. We describe its etiologies and guidelines for the prevention of this complication during nonsurgical endodontic treatment.