Thirty-six patients with chronic stable angina were studied before and after coronary artery bypass grafting (CABG) to assess the prevalence and prognostic implications of asymptomatic myocardial ischemia obtained by ambulatory monitoring. Ambulatory monitoring performed during medical therapy before CABG detected 66 episodes of transient ischemia, 54 (82%) being asymptomatic. All patients were asymptomatic or with minimal symptoms 3 months after CABG. Additional ambulatory monitoring was performed for 36 hours. There were 39 episodes of silent ischemia detected in the 12 patients of group 1, whereas no episodes of ST-segment shift occurred in the 24 patients of group 2. Coronary artery bypass grafting reduced the frequency of transient ischemia by 41% (p less than 0.05) compared with medical therapy, whereas the number of ischemic episodes in group 1 increased from 23 during medical therapy to 39 episodes after CABG (41%, p less than 0.05). During a follow-up of 9 months, 8 cardiac events occurred: 6 in group 1 comprising sudden death (1), revascularization (2), and angina (3) and 2 in group 2, including revascularization (1) and angina (1) (p = 0.005). Kaplan-Meier analysis demonstrated that asymptomatic myocardial ischemia was correlated with a significant cumulative probability of cardiac events (p less than 0.025) and multivariate analysis of 11 variables showed that silent ischemia was the most powerful predictor of cardiac events (p less than 0.005). Silent ischemia was a forerunner for angina pectoris in some patients, whereas angina did not occur during the follow-up period in others. This study does not reveal whether or not these patients are at higher risk for cardiac events during long-term follow-up.