Summary Background Intracoronary administration of an abciximab bolus during a primary percutaneous coronary intervention results in a high local drug concentration, improved perfusion, and reduction of infarct size compared with intravenous bolus application. However, the safety and efficacy of intracoronary versus standard intravenous bolus application in patients with ST-elevation myocardial infarction (STEMI) undergoing this intervention has not been tested in a large-scale clinical trial. Methods The AIDA STEMI trial was a randomised, open-label, multicentre trial. Patients presenting with STEMI in the previous 12 h with no contraindications for abciximab were randomly assigned in a 1:1 ratio by a central web-based randomisation system to intracoronary versus intravenous abciximab bolus (0·25 mg/kg bodyweight) during percutaneous coronary intervention with a subsequent 12 h intravenous infusion 0·125 μg/kg per min (maximum 10 μg/min). The primary endpoint was a composite of all-cause mortality, recurrent infarction, or new congestive heart failure within 90 days of randomisation. Secondary endpoints were the time to occurrence of the primary endpoint, each individual component of that endpoint, early ST-segment resolution, thrombolysis in myocardial infarction (TIMI) flow grade, and enzymatic infarct size. A masked central committee adjudicated the primary outcome and its components. Treatment allocation was not concealed from patients and investigators. This trial is registered with ClinicalTrials.gov , NCT00712101. Findings Between July, 2008, and April, 2011, 2065 patients were randomly assigned intracoronary abciximab (n=1032) or intravenous abciximab (n=1033). Intracoronary, as compared with intravenous abciximab, resulted in a similar rate of the primary composite clinical endpoint at 90 days in 1876 analysable patients (7·0% vs 7·6%; odds ratio OR 0·91; 95% CI 0·64–1·28; p=0·58). The incidence of death (4·5% vs 3·6%; 1·24; 0·78–1·97; p=0·36) and reinfarction (1·8% vs 1·8%; 1·0; 0·51–1·96; p=0·99) did not differ between the treatment groups, whereas less patients in the intracoronary group had new congestive heart failure (2·4% vs 4·1%; 0·57; 0·33–0·97; p=0·04). None of the secondary endpoints or safety measures differed significantly between groups. Interpretation In patients with STEMI undergoing primary percutaneous coronary intervention, intracoronary as compared to intravenous abciximab did not result in a difference in the combined endpoint of death, reinfarction, or congestive heart failure. Since intracoronary abciximab bolus administration is safe and might be related to reduced rates of congestive heart failure the intracoronary route might be preferred if abciximab is indicated. Funding Lilly, Germany. University of Leipzig—Heart Centre. University of Leipzig, Clinical Trial Centre Leipzig, supported by the Federal Ministry of Education and Research (BMBF).