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Background Acute basilar artery occlusion (BAO) carries a disability rate as high as 80% and its optimal treatment window remains a clinical controversy. While landmark studies such as ATTENTION (Endovascular Treatment for Acute Basilar-Artery Occlusion) and BAOCHE (Basilar Artery Occlusion Chinese Endovascular Trial) have confirmed the efficacy of endovascular treatment (EVT) within 24 hours, there is still a lack of high-level evidence supporting intervention strategies for patients beyond this time window.Aim The ANGEL (Acute Ischaemic Stroke Cooperation Group of Endovascular Treatment)-BAO trial aims to evaluate the efficacy and safety of EVT plus best medical management (BMM) compared with BMM alone in patients with BAO presenting within an extended time window of 24–72 hours from symptom onset (or last known well).Design This is a multicentre, prospective, randomised, open-label, blinded endpoint (PROBE) clinical trial. A total of 224 patients will be randomised in a 1:1 ratio to receive either EVT plus BMM or BMM alone. Key inclusion criteria include confirmed BAO, National Institutes of Health Stroke Scale (NIHSS)≥10, prestroke modified Rankin Scale (mRS) 0–2 and diffusion-weighted imaging evidence of limited infarct extent (posterior circulation Acute Stroke Prognosis Early CT Score (pc-ASPECTS)≥6 and Pons–Midbrain Index≤3).Outcome The primary efficacy outcome is the proportion of patients achieving an mRS score of 0–3 at 90 days. Secondary outcomes include mRS distribution, NIHSS scores, quality of life questionnaire (EuroQol 5-Dimension questionnaire) and recanalisation rates. Safety outcomes encompass symptomatic intracranial haemorrhage (per Heidelberg criteria) and mortality within 7 and 90 days.Discussion ANGEL-BAO addresses a critical gap in evidence regarding EVT for BAO in an extended time window (24–72 hours). By incorporating advanced imaging selection and standardised protocols, the trial aims to provide robust data to guide treatment strategies for this severe stroke subtype.Trial registration number NCT06101667.