Ambulatory electrocardiographic (ECG) monitoring of ischemia in patients with coronary artery disease (CAD) provides a new technique for the assessment of ischemic activity and the evaluation of therapies outside of the hospital. Numerous studies have demonstrated that the majority of patients with CAD have episodes of symptomatic and asymptomatic ST segment depression during routine daily activities. Rubidium-82 positron-emission tomographic studies have provided evidence for decreased myocardial perfusion during these episodes of ST segment depression. The prognostic importance of asymptomatic ischemia has been shown in patients with unstable angina to be a marker for early unfavorable cardiac events. Preliminary results suggest a poorer outcome for those patients with chronic stable angina who show episodes of ischemia as well. Ambulatory monitoring studies suggest that total ischemic activity may be underestimated by conventional testing. Whether all ischemic activity detected by ambulatory monitoring requires treatment awaits further study.