Shortly after a clot occludes a cerebral artery, cells in the infarct core—the most severely ischaemic area—start dying. The tissue surrounding the ischaemic core, the penumbra, is hypoperfused and at risk of progressing to infarction over minutes to hours, depending on the degree of ischaemia. The goal of treatment for acute stroke is to restore blood flow to these potentially salvageable areas as soon as possible. Thrombolysis is traditionally used, but in this issue Rodrigues and colleagues (doi:10.1136/bmj.i1754) summarise the evidence supporting mechanical thrombectomy.1 For more than 20 years the only treatment proven effective for patients with acute stroke was intravenous recombinant tissue plasminogen activator (rt-PA).2 3 When administered within 4.5 hours from symptom onset, rt-PA increases the odds of survival without clinically significant disability at three to six months.4 The treatment effect is time dependent: the odds ratio for a good outcome is 1.75 when rt-PA is given within three hours of the onset of symptoms, 1.26 when started between three and 4.5 hours, and a non-statistically significant 1.15 if administered beyond 4.5 hours.4 But this treatment has limitations. …