During acute occlusion of an epicardial vessel collaterals preserve the microvascular perfusion and limit the extent of myocardial damage. Pressure-derived collateral flow index (CFIp) assessed by intracoronary pressure measurement allow us to quantify collateral vessel development. The angiographic myocardial blush (MB) scores, based on the contrast dye density and washout in the infarcted myocardium, provide important information about microvascular perfusion after acute myocardial infarction (AMI). In this study we assessed the microvascular perfusion with MB and studied the relation between CFIp in patients with AMI who treated with thrombolytic therapy and TIMI grade III flow restored in the infarct related artery (IRA). Forty-one patients with AMI who were treated with thrombolytic therapy and underwent stent implantation (mean of 3 days after AMI) to the IRA were included in this study. After angiography, CFIp was calculated as the ratio of simultaneously measured coronary wedge pressure--central venous pressure (Pv) to mean aortic pressure--Pv. Myocardial blush was graded densitometrically based on visual assessment of the relative contrast opacification of the myocardial territory subtended by the infarct vessel. There was a statistically significant correlation between CFIp and post-stent myocardial blush grades (P < 0.01, r = 0.70). There was a significant difference in mean CFIp among myocardial blush grades implying that higher CFIp is associated with better MB (0.39 +/- 0.11 in grade 3, 0.32 +/- 0.10 in grade 2, 0.24 +/- 0.09 in grade 1, and 0.16 +/- 0.08 in grade 0, P < 0.01). Well developed collaterals can limit microvascular damage by preserving microvascular perfusion. A higher pressure-derived collateral flow index is associated with better tissue level perfusion as evidenced by the higher myocardial blush score.