Heart rate variability (HRV) was studied prospectively in 118 consecutive patients with acute myocardial infarction (AMI). HRV in AMI patients was significantly lower than that in normal controls, both at the 19th hour and the end of the 3rd week after the onset of AMI. Moreover, HRV at the end of 3rd week HRV in patients with cardiac death was also lower than that in the survivals. Measurements of ultra-low frequency power, total frequency power, very low frequency power, and the average of the standard deviation of every 5-minute period were independent predictors of cardiac death. All these 4 measurements have highly significant correlation (P < 0.01) and are better predictors of prognosis than other clinical variables. It is revealed that HRV at the end of 3rd week after the onset of AMI has much greater prognostic value than that at the initial period.