The aim of this study was to evaluate determinants of coronary blood flow following primary angioplasty (PA) in acute myocardial infarction (AMI). The corrected TIMI (thrombolysis in myocardial infarction) frame count and the TIMI flow grade were used as indexes of coronary blood flow, and its determinants were examined in 115 consecutive AMI patients who underwent PA (pain onset </= 12 hr). The following were validated as univariate predictors of slower corrected TIMI frame count: a lower pressure-derived farctional collateral flow (PDCF) index (P < 0.01), poor angiographic collaterals (P < 0.01), TIMI flow 0, 1 before PA (P < 0.05), and the presence of heavy thrombi (P < 0.01). The PDCF index and the presence of heavy thrombi were independent predictors of the corrected TIMI frame count. Likewise, the PDCF index (chi(2) = 12.9; P < 0.01) and the presence of heavy thombi (chi(2) = 11.4; P < 0.01) were independent predictors of TIMI 3 flow. In conclusion, collateral flow and the presence of thrombi are major determinants of coronary blood flow after PA in AMI.