Background and purposes: The relationship between post-stroke blood pressure (BP), collaterals and functional outcome in acute ischemic stroke patients is still controversial. The purpose of this study is to explore the influence of post-stroke baseline BP on collateral flow and functional outcome in acute ischemic patients with large vessel occlusion/stenosis. Methods: Patients identified with large vessel occlusion/stenosis with baseline multimodal computed tomography, clinical data, follow-up imaging, and clinical outcomes recruited from 2011 to 2017 in the International Stroke Perfusion Imaging Registry (INSPIRE) were included in this study. A modified Rankin Score of 0-1 at 90 days after stroke was defined as an excellent functional outcome. Collateral flow was quantified by the volume of tissue with severely delayed contrast transit (delay time>6 seconds) within the delay time>3 seconds perfusion lesion. The association between baseline BP, collateral flow and 3-month outcome were retrospectively investigated using multivariate models. Results: There were 306 patients included in this study. With every increase of 10mmHg in baseline systolic BP, the odds of achieving an excellent functional outcome decreased by 12% in multivariate analysis (OR 0.88, 95% CI 0.76-0.995, P=0.048). Conversely, every increment of 10mmHg in baseline BP (regardless of systolic, diastolic and mean BP) was associated with better collateral flow, and was also associated with reduced final infarct volumes (FIV). In subgroup analysis of patients with major reperfusion, higher BP was associated with decreased FIV. For patients without reperfusion, higher BP was associated with increased FIV and decreased odds of a favorable clinical outcome. Conclusions: Higher baseline BP in acute ischemic stroke patients with large vessel occlusion/stenosis was associated with better collateral flow. However, for patients without reperfusion, higher baseline BP was associated with increased infarct growth, and led to an unfavorable clinical outcome. To our knowledge, this is the first time that the association between BP and collaterals has been quantified by perfusion imaging, and this is also the first study to demonstrate that higher BP is correlated with infarct growth.