Background and Purpose— The study aims to determine whether volume transfer constant (K ^trans ) maps calculated from first-pass perfusion computed tomographic data are a biomarker of cerebral collateral circulation and predict the clinical outcome in acute ischemic stroke caused by proximal arterial occlusion. Methods— Consecutive patients with acute occlusion of the middle cerebral artery who received endovascular treatment were enrolled. Digital subtraction angiography, computed tomographic angiography with maximum intensity projection, and K ^trans maps were used to assess their collateral circulation. Agreement between different methods was evaluated using the χ ² tests. The correlations of various radiological and clinical outcomes with the collateral flow score, as determined from K ^trans maps, were calculated. Results— Seventy-five patients were included, comprising 39 women and 36 men, with a mean age of 65.3±14.6 years. Collateral flow score on K ^trans maps had the highest correlation with digital subtraction angiography (κ=0.8101; P =0.9796). Twenty-five patients had poor collateral circulation on K ^trans maps, 25 had intermediate collateral flow, 20 had good collateral flow, and 5 had excellent collateral flow. Better collateral circulation was associated with better clinical outcome ( P <0.0001). Conclusions— K ^trans maps extracted from standard first-pass perfusion computed tomography are correlated with collateral circulation status after acute proximal arterial occlusion and predictive of outcome.