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Introduction: It remains unknown who requires endovascular therapy in case of distal middle cerebral artery (MCA) occlusion.Hypothesis: Objective collateral assessment can identify acute ischemic stroke (AIS) patients with distal MCA occlusion refractory to intravenous tissue plasminogen (iv-tPA) alone.Methods: We Included patients 1) who were admitted to our institution from July 2009 to March 2017 within 4.5 hours of onset and diagnosed as AIS in the territory of the MCA, and 2) who underwent iv-tPA alone (0.6mg/kg). We assessed collateral status with time-intensity curves (TIC) produced by intravenous administration of gadolinium in bilateral MCA territories. We excluded patients with proximal MCA occlusion within 5 mm in length from the origin. According to the time to peak (TP) and the peak value (PV) of TIC comparing the affected side (a) with the contralateral side (c), we defined collateral status as following; Poor was defined as TP(a) minus TP(c) (TP delay) of 2 seconds or more and PV(a) divided by PV(c) (PV ratio) of less than 0.25, Moderate as TP delay of 2 seconds or more and PV ratio of 0.25 or more and of less than 0.75, Good as TP delay of less than 2 seconds or PV ratio of 0.75 or more. We evaluated association of collateral status with modified Rankin scale (mRS) scores at 3 months. We defined mRS score of 0 to 2 as favorable.Results: Sixty-six patients matched our criteria. A median NIHSS score was 12.5. Two patients had Poor collateral, 36 patients had Moderate and 28 patients had Good. Clinical outcome was favorable in none (0%) with Poor, in 19 (52.8%) with Moderate and in 18 (64.3%) with Good. Clinical outcome of all 5 patients with occlusion of opercular or cortical branches was favorable. Except for the 5 patients, among 34 patients with Moderate, outcome was favorable in 1 (50%) of 2 with NIHSS score of less than 5 and in 16 (50%) of 32 with NIHSS score of 5 or more, and among 25 patients with Good, outcome was favorable in all (100%) of 4 with NIHSS of less than 5 and in 11 (52%) of 21 with NIHSS of 5 or more.Conclusion: All patients with Poor collateral, half of the patients with Moderate collateral, about half of the patients with Good collateral coupled with NIHSS score of 5 or more are refractory to iv-tPA alone for distal MCA occlusion except opercular or cortical branches.