The frequency and duration of transient myocardial ischemia on Holter recordings, analyzed by the compact analog technique, were determined in 41 patients (all men, mean age 54) with unstable angina (33 with angiographic evidence). There were 781 episodes of ischemia: 392 (50%) with ST-segment depression, 242 (31%) with ST elevation, 45 (6%) with ST elevation and depression in different leads, 70 (9%) with pseudonormalization of T waves and 32 (4%) with T-wave augmentation. Ventricular arrhythmias were associated with 18% of the episodes. The mean duration of ischemic episodes was 14 minutes (range 30 seconds to almost 12 hours); most were less than 5 minutes. Only 154 (20%) of the 781 episodes of ischemia were associated with pain. Conversely, 77 episodes of chest pain were not associated with electrocardiographic changes. Analysis of the temporal sequence of heart rate during the development of ischemia (analyzed in 415 episodes) showed that in only 43 (10%) the heart rate at the beginning of ischemia was significantly (greater than 6 beats/min) higher than that at 5 minutes (baseline) before the onset of ischemia. At the peak of the ischemic abnormality, the mean heart rate increase was 10% and returned to baseline at the end of the ischemic episode. The data indicate that 80% of ischemic episodes in unstable angina are silent and over 90% are not triggered by increases in heart rate; apparently increased oxygen demand is an uncommon cause of ischemia in unstable angina. Although most of the episodes were short-lived, some were extremely protracted without the development of myocardial infarction. The findings are of therapeutic significance.