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Introduction Collateral circulation that determines the pace of infarct progression in large vessel occlusion (LVO) is regarded as stationary. Whether changes in collateral status occur during a stroke event remains unknown.Aims We assessed changes in collateral circulation during transfer for thrombectomy and their effect on functional outcome in anterior LVO strokes.Methods We analyzed 70 consecutive MCA occlusion stroke patients transferred from nine primary stroke centres between January 2017 and December 2018, who underwent repeated CTA upon arrival. Collateral circulation (Tan score, collateral percentage) was analyzed using artificial intelligence-assisted imaging software. Ordinal logistic regression was used to identify predictors of functional outcome.Results Collateral status was dynamic rather than stationary, generally worsening during transfer (M = -6.7%, SD = 27.64%). Collateral change emerged as a significant independent predictor of 90-day functional outcome (OR = 0.98, p = 0.008), with patients showing improvement in collaterals having better outcomes. Additionally, older age (OR = 1.04, p = 0.030), higher baseline NIHSS (OR = 1.20, p = 0.001), and longer stroke-to-reperfusion time (OR = 1.01, p = 0.014) were each significantly associated with worse 90-day functional outcomes. Collateral change remained significantly associated with 90-day functional outcomes (OR 0.97, p = 0.004) after adjusting for blood pressure.Conclusion Collateral status is not stationary during a stroke event and its improvement independently predicts good functional outcome highlighting the importance of therapeutic efforts to improve collateral circulation in LVO strokes.