The value of prehospital initiation of glycoprotein IIb/IIIa inhibitors remains a controversial issue. We sought to investigate whether in-ambulance initiation of abciximab in patients with ST-segment elevation myocardial infarction (STEMI) improves ST-segment elevation resolution (STR) after primary percutaneous coronary intervention (PCI). MISTRAL (Myocardial Infarction with ST-elevation Treated by Primary Percutaneous Intervention Facilitated by Early Reopro Administration in Alsace) is a prospective, randomized, double-blind study. Two hundred and fifty-six patients with acute STEMI were allocated to receive abciximab either in the ambulance (ambulance group, n=127) or in the catheterization laboratory (hospital group, n=129). The primary end point was complete (>70%) STR after PCI. Complete STR was not significantly different between the 2 groups (before PCI, 21.6% versus 15.5%, P=0.28; after PCI, 70.3% versus 65.8%, P=0.49). Thrombolysis In Myocardial Infarction (TIMI) 2 to 3 flow rates before PCI tended to be higher in the ambulance group (46.8% versus 35%, P=0.08) but not after PCI (70.3% versus 65.8%, P=0.49). Slow flow tended to be lower (5.6% versus 13.4%, P=0.07), and distal embolization occurred significantly less often in the ambulance group (8.1% versus 21.1%, P=0.008). One- and 6-month major adverse cardiac event rates were low and similar in both groups. Early ambulance administration of abciximab in STEMI did not improve either STR or TIMI flow rate after PCI. However, it tended to improve TIMI flow pre-PCI and decreased distal embolization during procedure. Larger studies are needed to confirm these results.