The predictive value of Killip classification of acute myocardial infarction (AMI) in patients undergoing percutaneous coronary intervention (PCI) is not well established. We performed a pooled analysis of 2,654 patients with AMI enrolled in 3 primary angioplasty trials. Of these, 2,305 patients were class I, 302 were class II, and 47 were class III (class IV patients were excluded). Univariate and multivariate analyses were performed to determine if Killip class at admission was a predictor of in-hospital and 6-month mortality. Higher Killip classification was associated with greater in-hospital (2.4%, 7%, and 19% for class I, II, and III, respectively) and 6-month mortality (4%, 10%, and 28% for class I, II, and III, respectively). Higher Killip class was associated with increased age (p <0.001), history of diabetes (p <0.02), lower systolic blood pressure and higher heart rate at presentation (p <0.0001 for both), more 3-vessel disease (p <0.001), lower left ventricular ejection fraction (p <0.0001), and higher peak creatine phosphokinase (p <0.0001). With each increasing Killip class, there was an increased need for an intra-aortic balloon counterpulsation (p <0.001) and greater incidence of renal failure (p <0.001), major arrhythmia (p <0.001), and major bleeding (p <0.001). After controlling for potential confounding variables, Killip classification remained a multivariate predictor of mortality at both time end points. Killip classification at hospital admission remains a simple and useful independent predictor of in-hospital and 6-month mortality in patients with AMI who are undergoing primary PCI.