Background: The validity of angiographic collateral grade according to the Rentrop classification during acute myocardial infarction (AMI) and its relation to flow in occluded coronary arteries before angioplasty have never been evaluated. Methods: We assessed the validity of the angiographic collateral grade according to Rentrop classification in relation to collateral pressure and flow beyond occluded coronary arteries during AMI. Pressure distal to coronary artery occlusions before balloon dilatation was measured in 111 patients undergoing angioplasty for AMI. We calculated the collateral flow index (CFI) and compared it to observed Rentrop grade and measured creatine kinase sum. Results: The values of pressure distal to coronary artery occlusions with respect to collateral grades 0 to 3 were 33 ± 12, 37 ± 13, 42 ± 10, and 60 ± 14 mm Hg (P < .0001). Overall CFI was 0.35 ± 0.13 (median 0.33), with CFI values of 0.3 ± 0.13, 0.33 ± 0.13, 0.39 ± 0.1, and 0.57 ± 0.2 for collateral grades 0 to 3, respectively (P < .0001). Larger creatine kinase elevation (P < .016) and higher white blood cell count (P < .022) were recorded in the lowest tertile CFI compared with highest tertile CFI group; but no difference in the global, regional, or infarct-related regional left ventricular contraction was found. Conclusions: These observations demonstrate that the Rentrop classification is valid in AMI patients with occluded coronary arteries and that collaterals are recruited acutely. These collaterals, whose pressure-derived CFI during AMI was shown for the first time to be higher than its value reported in chronic conditions, may limit the immediate myocardial damage or the systemic inflammatory response. No impact on global or regional cardiac contraction was detected in a population where most patients were treated early.