The relative prognostic significance of ischemic ST changes on two-lead continuous electrocardiographic (Holter) monitoring in 103 high-risk postinfarction patients was examined. Ischemic ST changes were detected in 30 patients, with a median number of five episodes per day and median total daily duration of 157 minutes. Only one third of these patients reported any angina in the hospital, and 28 of the 30 patients had silent ST changes on Holter monitoring. The remaining 73 patients had no ischemic ST changes on Holter monitoring. At one year, nine (30%) of 30 patients with ischemic ST changes were dead vs only eight (11%) of 73 patients without such changes. Multivariate Cox's hazard function analysis on 18 variables, including age, type of infarction, Lown and Killip class, ejection fraction, and medications, showed that the presence of ST changes on Holter monitoring was a significant predictive variable for one-year mortality in the overall study population and particularly in the subgroup of 59 patients who could not undergo early exercise treadmill testing. Thus, ischemic ST changes on Holter monitoring, the majority of which are silent, occur in nearly one third of high-risk postinfarction patients and are significantly associated with one-year mortality.