Background. - Compared with administration in the catheterization laboratory, early treatment with glycoprotein IIb/IIIa inhibitors provides benefits to patients with ST-segment elevation myocardial infarction who undergo primary percutaneous intervention. Whether this benefit is maintained on top of a 600 mg loading dose of clopidogrel is unknown. Methods. - In a multicentre, controlled, randomized study, 320 patients with ST-segment elevation myocardial infarction undergoing primary percutaneous coronary intervention received a high-dose bolus of tirofiban given either in the ambulance (prehospital group) or in the catheterization laboratory. The primary endpoint was a TIMI flow grade 2-3 of the infarct-related vessel at initial angiography. Secondary endpoints included ST-segment resolution 1 h after percutaneous coronary intervention and peak serum troponin I concentration. Results. - Tirofiban was administered 48 (95% confidence interval 21.4-75.0) min earlier in the prehospital group. At initial angiography, the combined incidence of TIMI 2-3 flow was 39.7% in the catheterization-laboratory group and 44.2% in the prehospital group (p = 0.45). No difference was found on postpercutaneous intervention angiography or peak troponin concentration. Complete ST-segment resolution 60 min after the start of intervention was 55.4% in the catheterization-laboratory group and 52.6% in the prehospital group (p = 0.32). Conclusion. - Prehospital initiation of high-dose bolus tirofiban did not improve significantly initial TIMI 2 or 3 flow of the infarct-related artery or complete ST-segment resolution after coronary intervention compared with initiation of tirofiban in the catheterization laboratory