Two hundred sixty-seven operations performed consecutively at the National Cancer Center Hospital, Tokyo, in 1990 were reviewed to define the prevalence, type, clinical course, and, especially, risk factors for arrhythmias after thoracic operations. Arrhythmias were identified in 63 operations (23.6%) and were more prevalent in several subgroups of patients than in others--those with lung cancer, pneumonectomy, mediastinal lymph node dissection, and those older than 70 years of age. Supraventricular tachycardias, of which atrial fibrillation was the most common, comprised 95.3% of the cases; bradyarrhythmia and ventricular ectopic beats were seen in only three and four cases, respectively. Arrhythmias were most likely to develop on the second day after the operation. Eighty percent of the arrhythmias disappeared within 3 days after onset, and sinus rhythm was finally restored with digitalis or other antiarrhythmic drugs in all patients except one, who had a myocardial infarction. Arrhythmias were not the direct cause of any of the seven in-hospital deaths. A multivariate analysis of 16 variables revealed that age and extent of pulmonary resection were significant risk factors. Despite these significant risk factors, arrhythmias after thoracic operations could be managed without special prophylaxis and were not closely related to higher mortality.