CONTEXT:Approximately 0.5% of all patients who undergo exercise testing develop a transient left bundle-branch block (LBBB) during exercise, but its prognostic significance is unclear. OBJECTIVE:To determine whether exercise-induced LBBB is an independent predictor of mortality and cardiac morbidity. DESIGN:Matched control cohort study. Between September 1990 and February 10, 1994, 17277 exercise stress tests were performed on patients. SETTING:Tertiary care, academic medical center. PATIENTS:From the cohort, 70 cases of exercise-induced LBBB were identified. The controls comprised 70 individuals without LBBB at rest or during exercise that matched the 70 cases based on age, test date, sex, prior history of coronary artery disease, hypertension, diabetes, smoking, and beta-blocker use. MAIN OUTCOME MEASURES:All-cause mortality, percutaneous coronary intervention, open heart surgery, nonfatal myocardial infarction, documented symptomatic or sustained ventricular tachydysrhythmia, or implantation of a permanent pacemaker or an implantable cardiac defibrillator. RESULTS:A total of 37 events (28 events from the exercise-induced LBBB cases and 9 from the control cohort) occurred in 25 patients (17 exercise-induced LBBB patients and 8 control patients) during a mean follow-up period of 3.7 (0.9 years) (median, 3.8 years [range, 0.9-5.2 years]). There were 7 deaths, of which 5 occurred among patients with exercise-induced LBBB. Four-year Kaplan-Meier event rates were 19% among exercise-induced LBBB patients and 10% among controls (log-rank chi2, 5.2; P=.02). After further adjusting for small differences in age, exercise-induced LBBB remained associated with a higher risk of primary events (adjusted relative risk, 2.78; 95% confidence interval, 1.16-6.65; P=.02). CONCLUSION:Exercise-induced LBBB independently predicts a higher risk of death and major cardiac events.