We have used continuous and concurrent monitoring of arterial oxygen saturation (SpO2) and ECG to study the relationship between hypoxaemia and silent myocardial ischaemia in the perioperative period in 11 patients with cardiovascular disease. Ischaemic and hypoxaemic events occurred in all patients. Many events were shortlived and occurred independently of each other. However, our results suggest a close correlation between the duration of hypoxaemia and myocardial ischaemia. Ischaemia is more likely to occur if an episode of hypoxaemia is prolonged (beyond 5 min; P less than 0.01, chi square) and severe (SpO2 less than 85%; P less than 0.05, chi square).