The degree of reduction in heart rate variability (HRV) after myocardial infarction has been shown to have prognostic significance, but HRV has not been studied extensively in patients with unstable angina. We assessed spectral and nonspectral measurements of HRV in 52 patients with unstable angina, 52 patients with acute myocardial infarction, and 41 normal subjects. The spectral bands of 0.04 to 0.15 Hz (low frequency), 0.15 to 0.4 (high frequency), and nonspectral parameters SDNN, SDANN, SDNN index, rMSSD, and pNN50 were calculated from continuous 24-hour ECGs. All measures of HRV were reduced in patients with acute coronary syndromes compared to normal controls (p < 0.001), and there was no significant difference in measure of HRV between unstable angina and myocardial infarction patients. In patients with unstable angina who stabilized after admission, HRV had increased by the second 24 hours of monitoring. In contrast, HRV was further depressed in patients who had episodes of chest pain or transient ST-segment depression during the second 24 hours. rMSSD, pNN50, and SDNN index were lower in patients with unstable angina who had transient silent ischemia compared to those without silent ischemia. Of the patients with unstable angina, 4 died and 1 had nonfatal acute myocardial infarction within 11 months. HRV was lower in these patients than in patients without further cardiac events.