A case report of an endotracheal tube fire occurring during electrodissection tonsillectomy is presented. The authors believe that this incident occurred because a retrograde leak of ventilating gases around an uncuffed endotracheal tube during positive-pressure ventilation produced a high oxygen concentration in the mouth, allowing indirect ignition of the tube. In vitro testing supported this hypothesis. Ignition tests on polyvinylchloride endotracheal tubes using electrocautery in various oxygen concentrations were performed. As oxygen concentration increased, the endotracheal tube could be moved further from the cautery and still allow ignition of the tube. At 52% oxygen, with the cautery set at 25-W coagulation current, the endotracheal tube could not be ignited. Recommendations to prevent a recurrence of this incident are included.