Angina is not a very sensitive indicator of myocardial ischaemia. In patients with coronary disease 75 percent of ischaemic episodes are asymptomatic. Holter monitoring enables such silent episodes to be detected in daily life, this method becoming more sensitive when pursued for several days. The procedure is facilitated by a new generation of Holter recorders fitted with microprocessors that digitalize electrocardiograms. Silent episodes occur in the same circumstances as painful episodes, with a peak of incidence between 6 a.m. and noon, but they are often somewhat shorter. In patients with stable angina, as in those with unstable angina and after infarction, silent ischaemia is of poor prognosis. Holter monitoring therefore is useful in patients with known coronary disease to identify subjects at risk and to evaluate the effectiveness of anti-ischaemic treatments.