A total of 238 patients undergoing resection of lung cancer were studied for the occurrence of postoperative arrhythmia. Transient arrhythmia was observed in 43 of them (18.1%). Ninety-one percent of 43 patients developed atrial fibrillation (Af), even though no arrhythmia was noted on ECG in any patient preoperatively. Cardiac dysrhythmia occurred 5.2 +/- 3.8 days after operation and lasted for 1.3 +/- 0.9 days (mean +/- SD). There was a significant difference (p less than 0.05) in the incidence of postoperative arrhythmia between the male group (39/188 = 21%) and the female group (4/50 = 8%), however the cause of a difference is unknown. The incidence was higher in patients undergoing pneumonectomy than in those undergoing lobectomy. Patients, who suffered from postoperative arrhythmia, has significantly low values on pulmonary function test (FEV1.0% = 68.7%) preoperatively. The increased cardiac load after the reduction of the pulmonary vascular bed was regarded as the most important factor of arrhythmia. Prophylactic administration of digoxin was performed in another 61 male patients after resection of lung cancer. And it was found to be effective in decreasing the incidence of postoperative atrial fibrillation (5/61 = 8%).