Cardiac tachydysrhythmias occurred in 53 (22 percent) of 236 consecutive patients undergoing pneumonectomy. All patients had preoperative electrocardiograms which showed normal sinus rhythm. Patients did not receive digitalis before surgery. Atrial fibrillation was the most common dysrhythmia (64 percent; 34/53), followed by supraventricular tachycardia (23 percent; 12/53) and atrial flutter (13 percent; 7/53). No episodes of ventricular tachycardia were documented. Elevated concentrations of cardiac enzymes were associated with 12 (28 percent) of 43 tachydysrhythmias. Recurrent or persistent dysrhythmias were documented in 29 (55 percent) of 53 patients despite medical management or electrocardioversion (or both). Thirty-one percent (9/29) of these patients subsequently died during their hospitalization. There was no correlation between standard preoperative pulmonary function tests and the incidence of postoperative dysrhythmia. In addition, there was no correlation of dysrhythmia with postoperative diagnoses, surgical staging for lung cancer, postoperative arterial blood gas levels, or the fact that a completion pneumonectomy or chest wall resection was undertaken. An increased incidence of tachydysrhythmia was noted in patients undergoing intrapericardial dissections and those who developed postoperative interstitial or perihilar pulmonary edema. Twenty-five percent (13) of the patients experiencing tachydysrhythmias died within 30 days following their pneumonectomy. We conclude that tachydysrhythmias after pneumonectomy are associated with significant mortality, have poor correlation to preoperative pulmonary function, and occur more frequently following intrapericardial dissection and in patients who develop postoperative interstitial pulmonary edema or perihilar pulmonary edema.