To determine whether an increased QRS duration in the absence of bundle branch block (BBB) on the presenting electrocardiogram of patients with acute myocardial infarction (AMI) is associated with decreased survival, we retrospectively reviewed 781 consecutive patients admitted to the coronary care unit of our institution with AMI without BBB between 1988 and 1998. In patients with ST elevation AMI (n = 412), the groups with QRS duration ≥100 ms and <100 ms had similar survival. Conversely, in patients with non-ST elevation AMI (n = 369), in-hospital, 1-, 3-, and 5-year survival was 84.4%, 75.6%, 66.7%, and 52.2%, respectively, in the group with QRS ≥100 ms compared with 95.4%, 89.2%, 83.8%, and 74.3%, respectively, in the group with QRS <100 ms (p <0.01, log-rank test). In patients with non-ST elevation AMI, those with QRS duration ≥100 ms were more likely to be men, to have had a prior AMI that healed, to be in Killip class II, III, or IV, and to have lower ejection fraction than patients with QRS duration <100 ms. After adjusting for age, sex, prior AMI or stroke, heart rate, and Killip class on admission, QRS duration ≥100 ms was independently associated with in-hospital and overall mortality in patients with non-ST elevation AMI. QRS duration ≥100 ms in the absence of BBB is an independent predictor of increased mortality in patients with non-ST elevation AMI.