Background The outcome of patients with previous coronary artery bypass grafting (CABG) undergoing primary percutaneous coronary intervention (PCI) for the treatment of acute myocardial infarction (AMI) is unclear. We sought to assess the outcome of patients with prior CABG undergoing primary PCI for the treatment of AMI. Methods and Results Between 1991 and 1997, 1072 patients with AMI underwent primary PCI without antecedent thrombolytic therapy at the Mayo Clinic. There were 128 patients with previous CABG and 944 without previous CABG. Patients with previous CABG were further subdivided according to the treated vessel: native vessels (n = 65) and bypass graft (n = 63). Clinical and angiographic characteristics and 30-day and 1-year outcomes were evaluated. Patients with previous CABG were significantly older and had a higher incidence of diabetes, hypertension, and hypercholesterolemia. They had a lower left ventricular ejection fraction and were also more likely to have congestive heart failure. After 1 year of follow-up, adverse cardiac events (death, MI, CABG, or repeat PCI) were significantly greater in patients with prior CABG (49.2% vs 35.9%, P =.04). With use of multivariate logistic regression analysis to adjust for differences in baseline characteristics, the treatment of vein graft was independently associated with adverse cardiac events (relative risk 1.48 [95% confidence interval 1.07-2.03], P =.02), but a history of prior CABG itself was not (relative risk 1.22 [95% confidence interval 0.96-1.56], P =.11). Conclusions Primary PCI for AMI in patients with previous CABG is associated with higher adverse events largely attributable to adverse baseline clinical characteristics and the treatment of a vein graft. (Am Heart J 2001;142:452-9.)