The prognosis of high-risk patients with non-ST-elevation acute coronary syndrome can be improved by invasive therapy. Taking into account the large number of patients with symptoms suggestive of acute coronary syndrome, the heterogeneity of the population and the increased risk of events shortly after the onset of symptoms, a strategy for initial evaluation and treatment is essential. The electrocardiogram (ECG) is the most accessible and widely used diagnostic tool for patients with symptoms suggestive of acute myocardial ischaemia. The ECG is almost never normal during episodes of rest angina. A specific ECG pattern, transient ST-segment depression and negative T waves maximally in leads V4-5, is associated to left main or severe triple vessel disease, and should alert the treating physician to admit the patient for immediate invasive evaluation. The ECG finding is a result of severe wide-spread subendocardial ischaemia.