In patients with left bundle branch block (LBBB) and acute chest pain, the association between the clinical presentation and the diagnosis of myocardial infarction has not been investigated. We sought to identify features in the clinical history of patients with LBBB and acute cardiopulmonary symptoms that predict myocardial infarction among candidates for reperfusion therapy. We retrospectively studied a consecutive cohort of 75 patients (94 presentations) who presented to a university emergency department from 1994 to 1997 with LBBB on initial electrocardiogram (ECG) and acute chest pain of >/=20 min duration or acute pulmonary edema. Among the 94 presentations meeting criteria for the cohort, 26 (28%) had confirmed myocardial infarction. Coronary heart disease risk factors, past cardiac history, prior LBBB on the ECG, and presenting symptoms did not predict whether patients were having myocardial infarction. The clinical history was not effective at distinguishing LBBB patients with myocardial infarction among patients who appeared to be candidates for acute reperfusion therapy.