Continuous monitoring of the electrocardiogram in patients with angina pectoris and coronary artery disease (CAD) has shown episodes of asymptomatic ST-segment depression, suggesting frequent silent myocardial ischemia during normal daily life. Interpretation of this new finding depends on whether similar changes occur in normal subjects. Frequency-modulated ambulatory electrocardiographic recordings were performed in 80 asymptomatic normal volunteers (20 from each decade between 20 and 50 years and, 20 more than 50 years old) and in 20 patients with noncardiac pain, negative exercise and provocative tests and angiographically normal coronary arteries. Treadmill exercise testing was performed in all subjects more than 40 years old. Episodes of T-wave change were identified in 53 subjects. Five subjects younger than 40 years had episodes of ST elevation that were prolonged; they usually occurred at night. In 3 patients they could be reproduced by postural change. Only 2 subjects, both older than 40 years had planar ST depression during tachycardia; one of these subjects had a positive exercise test response. No patient with normal coronary arteries had significant ST depression. Tachycardia was frequently associated with upsloping ST depression (36%), which was more common in younger subjects: Five subjects also showed isolated single complexes with ST depression during baseline instability. With use of frequency-modulated recordings, transient ST depression of 0.1 mV or greater that lasted 80 ms or longer and more than 30 seconds in duration, was rare in a normal population. This finding supports the use of this signal to follow the activity of CAD out of the hospital, specifically in patients with typical angina and proved CAD.