This report describes a rare case of a patient undergoing implantation of a cardioverter defibrillator in whom a right pneumothorax resulting from an endocardial screw-in atrial lead was observed. Pericardial effusion, substantial changes in lead impedance, andpacing and sensing thresholds were not observed. Pneumothorax disappeared spontaneously and has not reoccurred during follow-up with conservative therapy alone.