The aim of this study was to evaluate the impact of pretreatment quality of collaterals, involving velocity and extent of collateral filling, on recanalization after intravenous thrombolysis (IVT). A retrospective analysis was performed of 66 patients with acute middle cerebral artery (MCA) M1 segment occlusion who underwent MR perfusion (MRP) imaging before IVT. The velocity of collateral filling was defined as arrival time delay (ATD) of contrast bolus to Sylvian fissure between the normal and the affected hemisphere. The extent of collateral filling was assessed according to the Alberta Stroke Program Early CT (ASPECT) score on temporally fused maximum intensity projections (tMIP). Arterial occlusive lesion (AOL) score was used to assess the degree of arterial recanalization. ATD (OR = 0.775, 95% CI = 0.626-0.960, p = 0.020), but not tMIP-ASPECT score (OR = 1.073, 95% CI = 0.820-1.405, p = 0.607), was independently associated with recanalization (AOL score of 2 and 3) at 24 hours after IVT. When recanalization was achieved, hemorrhagic transformation (HT) occurred more frequently in patients with slow collaterals (ATD ≥ 2.3 seconds) than those with rapid collaterals (ATD < 2.3 seconds) (88.9% vs 38.1%, p = 0.011). In conclusion, the velocity of collaterals related to recanalization, which may guide the decision-making of revascularization therapy in acute ischemic stroke.