Silent myocardial ischemia is diagnosed by several different techniques and has been documented in all the anginal syndromes. In addition to other factors, its presence may be related to increased pain threshold and increased pain tolerance. Although some patients with painless ischemia may have less extensive coronary artery disease, cumulative evidence indicates that silent myocardial ischemia does not necessarily signify a lesser degree of cardiac ischemia or a less severe coronary abnormality. As judged by ambulatory monitoring studies, it shows circadian variation; occurs more frequently than symptomatic ischemia; and appears to depend, in large part, on activation of the sympathetic nervous system. Frequent silent ischemic events during ambulatory monitoring are worrisome because they reflect the disease "activity" of single or multiple coronary atherosclerotic lesions. Thus, there may be a direct association between the severity of ischemia seen during Holter monitoring, the extent of underlying coronary artery disease or disease activity, and prognosis. When diagnosed by exercise testing, silent myocardial ischemia may be associated with significant coronary involvement. In this regard, patients with three vessel coronary disease, impaired left ventricular function, and silent ischemia during stress testing should benefit from coronary revascularization. Compared with symptomatic patients, other evidence suggests that patients with exercise-induced asymptomatic ischemia have at least the same or perhaps even a worse outlook; this may be related to the lack of symptoms that would prompt evaluation and therapy. Awareness of the possibility of silent myocardial ischemia and use of commonly available tests, both to establish its presence and severity and to guide treatment, are emerging as new clinical goals. Further data, however, are necessary to determine how vigorously this should be pursued in different patient subgroups. In association with unstable angina or post-myocardial infarction, the added risk of silent myocardial ischemia warrants a more aggressive approach.