Twenty-four hour electrocardiographic (Holter) monitoring has been adopted in primary care for the assessment of patients with suspected cardiac arrhythmias. This article examines the clinical use of the Holter monitoring technique, outlines the range of arrhythmias recorded in healthy adults, and evaluates the merit of the investigation in a primary-care clinical setting. Holter monitoring is most valuable in assessing symptomatic patients with known coronary artery disease of in the postinfarction period. Although frequent and complex arrhythmias in such patients may be associated with sudden death, it is not yet known whether treatment decreases mortality.