We have studied myocardial ischaemia, heart rate and arrhythmia in 82 patients undergoing elective thoracotomy. Myocardial ischaemia was detected using a microprocessor-based surveillance system programmed to record leads V2 and V5. Patients were monitored on the day before and for up to 72 h after surgery. The total monitoring time was 5158 h. The incidence of silent myocardial ischaemia before operation was 11% (nine of 82). This increased to 24% (20 of 82) after operation. Postoperative myocardial ischaemia was associated with preoperative myocardial ischaemia in six patients. Before operation, the mean duration of myocardial ischaemia was 0.31 min per hour of monitoring. After operation, this increased to 1.36 min per hour of monitoring (P < 0.05). For the whole population, mean heart rate before operation was 74 beat min-1 and increased to 84 beat min-1 after operation (P < 0.01). Patients with ischaemia had a mean heart rate of 92.8 beat min-1 after operation compared with those with no ischaemia whose mean heart rate was unchanged at 81.8 beat min-1 (P < 0.05). The incidence of atrial tachyarrhythmia increased from one patient before operation to 12 patients after operation (P < 0.01). Atrial tachyarrhythmia was not associated with postoperative myocardial ischaemia. Nine patients had an adverse operative outcome; two had non-fatal myocardial infarction and seven died. Postoperative myocardial ischaemia was associated with adverse outcomes (P < 0.05).