Background: Ischemic stroke lesion volume at follow-up is an important surrogate outcome for acute stroke trials. We aimed to assess which differences in 48-hour lesion volume translate into meaningful clinical differences., Methods: We used pooled data from 7 trials investigating the efficacy of endovascular treatment for anterior circulation large vessel occlusion in acute ischemic stroke. We assessed 48-hour lesion volume follow-up computed tomography or magnetic resonance imaging. The primary outcome was a good functional outcome, defined as modified Rankin Scale (mRS) scores of 0 to 2. We performed multivariable logistic regression to predict the probability of achieving mRS scores of 0 to 2 and determined the differences in 48-hour lesion volume that correspond to a change of 1%, 5%, and 10% in the adjusted probability of achieving mRS scores of 0 to 2., Results: In total, 1665/1766 (94.2%) patients (median age, 68 [interquartile range, 57-76] years, 781 [46.9%] female) had information on follow-up ischemic lesion volume. Computed tomography was used for follow-up imaging in 83% of patients. The median 48-hour lesion volume was 41 (interquartile range, 14-120) mL. We observed a linear relationship between 48-hour lesion volume and mRS scores of 0 to 2 for adjusted probabilities between 65% and 20%/volumes <80 mL, although the curve sloped off for lower mRS scores of 0-2 probabilities/higher volumes. The median differences in 48-hour lesion volume associated with a 1%, 5%, and 10% increase in the probability of mRS scores of 0 to 2 for volumes <80 mL were 2 (interquartile range, 2-3), 10 (9-11), and 20 (18-23) mL, respectively. We found comparable associations when assessing computed tomography and magnetic resonance imaging separately., Conclusions: A difference of 2, 10, and 20 mL in 48-hour lesion volume, respectively, is associated with a 1%, 5%, and 10% absolute increase in the probability of achieving good functional outcome. These results can inform the design of future stroke trials that use 48-hour lesion volume as the primary outcome., Competing Interests: Disclosures Dr Rinkel is supported by the Niels Stensen Fellowship. Dr Ospel reports consulting fees from NICOLAB. Dr Brown reports consulting fees from the University of Calgary during the conduct of the HERMES (Highly Effective Reperfusion Evaluated in Multiple Endovascular Stroke Trials) collaboration and reports consulting fees from MicroVention. Dr Goyal reports that Medtronic provided a grant to the University of Calgary and consulting fees from Medtronic, Philips, Microvention, Stryker, and Mentice. Dr Muir reports a grant from Boehringer-Ingelheim paid to the institution for support of the ATTEST-2 (Alteplase-Tenecteplase Trial Evaluation for Stroke Thrombolysis); consulting fees from Boehringer-Ingelheim, AbbVie, and Biogen; and British Heart Foundation-funded data safety and monitoring board participation (ARREST [Advanced Reperfusion Strategies for Patients With Out-of-Hospital Cardiac Arrest and Refractory Ventricular Fibrillation]). Dr White reports unrestricted grants from Stryker, Medtronic (Covidien), and Penumbra paid to the institution. Dr Jovin reports honoraria from Stryker, Silk Road Medical, Blockade Medical, FreeOx Biomedical, Route 92, Neurotrauma Science, vizai, Corindus, Anaconda, Medtronic, Contego, and Methinks as a consultant; and has consulted for Cerenovus as a steering committee member and Stryker Neurovascular as a principal investigator of DAWN (Clinical Mismatch in the Triage of Wake Up and Late Presenting Strokes Undergoing Neurointervention With Trevo) and AURORA (Analysis of Pooled Data From Randomized Studies of Thrombectomy More Than 6 Hours After Last Known Well). Dr Hill reports grant/contracts from Biogen Inc, Boehringer-Ingelheim, Canadian Institutes of Health Research, Micorvention, Medtronic, NoNO, Inc, consultant for Brainsgate Ltd, End point review committee at Merck, employment at the University of Calgary, and patent for stroke imaging software with Circle NVI. Dr Mitchell reports speaking engagements from Stryker and Medtronic and research institutional support from Stryker Neurovascular and Medtronic. Dr Saver is a consultant for Johnson & Johnson Health Care Systems, Inc, Medtronic USA, Inc, Boehringer-Ingelheim, MIVI Neuroscience, Stryker Corporation, Abbott Laboratories, Aeromics, Biogen, BrainQ, Brainsgate, CSL Behring, Roche, stock options (Rapid Medical, MindRhythm, Neuronics Medical). Dr Muir reports consulting for AbbVie, Biogen, Boehringer-Ingelheim, Woolsey Pharma and IschemiaView. Dr Jovin reports stock options from Anaconda, Freeox Biotech, Kandu, Galaxy, Methinks, Route92, vizai, consulting for Contego Medical, Inc, data and safety monitoring at Johnson & Johnson Cerenovus, grant/contract from Medtronic USA, Inc, Stryker and employment at Cooper University Healthcare. Dr Demchuk reports compensation from Boehringer-Ingelheim, HLS Therapeutics, Hoffman-Laroche, Medtronic, Nova-Signal, and Servier for consultant services, others from AstraZeneca, Novo Nordisk, and Pfizer, data safety monitoring board participation with Philips, and stock and patent for stroke imaging software with Circle NVI. Dr Majoie reports grants from the Dutch Heart Foundation, European Commission, Health Evaluation Netherlands, Stichting Toegepast Wetenschappelijk Instituut voor Neuro-modulatie Foundation (TWIN Foundation), and Stryker (all paid to institution); and is a (minority interest) shareholder of Nicolab. Dr Dippel reports unrestricted research grants from the Dutch Heart Foundation, Brain Foundation Netherlands, The Netherlands Organisation for Health Research and Development, Health Holland Top Sector Life Sciences and Health, and unrestricted grants from Medtronic, Penumbra, Stryker, Thrombolytic Science, and Cerenovus (all paid to institution).