Objectives: We investigated the incidence, predictors, and prognosis of gastrointestinal bleeding (GIB) in patients undergoing primary percutaneous coronary intervention (PCI) for ST-elevation myocardial infarction (STEMI). Study design: We reviewed 2,541 consecutive patients (2,111 males, 430 females; mean age 56.5 +/- 11.8 years) who underwent primary PCI for STEMI. Data on clinical, angiographic findings, and in-hospital outcomes were collected. Gastrointestinal bleeding was defined as apparent upper or lower GIB or melena requiring cessation of antiplatelet or anticoagulant therapy and administration or erythrocyte infusion. Results: Gastrointestinal bleeding was observed in 27 patients (1.1%). Compared to 2,514 patients without GIB, patients with GIB were older (65.9 +/- 13.5 years vs. 56.4 +/- 11.8 years; p<0.001), exhibited higher frequencies of female gender (p=0.016), renal failure (p<0.001), and admission anemia (p<0.001), and had a lower procedural success rate (77.9% vs. 91.5%;p=0.02). The development of GIB was associated with significantly higher in-hospital mortality (18.5% vs. 2.9%; p<0.001), longer hospital stay (13.1 +/- 6.8 days vs. 7.0 +/- 3.7 days, p=0.02), and increased inotropic requirement (37% vs. 6.7%; p<0.001). In multivariate analysis, inotropic requirement (OR 4.17, 95% CI 1.7-10.4; p=0.002), age above 70 years (OR 3.33, 95% CI 1.4-8.0; p=0.007), and glomerular filtration rate lower than 60 ml/min/1.73 m(2) (OR 2.96, 95% CI 1.2-7.4; p=0.02) were independent predictors of in-hospital GIB. Conclusion: The development of GIB is not an uncommon complication after primary PCI for STEMI. These patients have a prolonged hospital stay and increased in-hospital mortality. Increased intropic requirement, age above 70 years, and impaired renal function are independent predictors of this complication.