Silent ischaemia (objective signs of myocardial ischaemia without symptoms) can be diagnosed using a conventional exercise-test or ambulatory Holter monitoring. Silent ischaemia is a frequent phenomenon in patients with ischaemic heart disease, i.e. patients with angina pectoris or previous myocardial infarction. The reason why ischaemia is symptomatic in some cases, and asymptomatic in others is unknown. Different possible mechanisms are discussed. Myocardial ischaemia, symptomatic or not is accompanied by a compromised function of the left ventricle, including reduced ejection fraction during exercise. In selected groups of patients, silent ischaemia is related to an impaired prognosis, while it does not seem to carry any prognostic information in other groups of patients. Silent ischaemia can be treated/reduced using antianginal medication or revascularization, but for the time being it is not known if treatment can improve prognosis. Studies concerning the latter are under way.