Patients waking up with stroke symptoms are generally excluded from intravenous thrombolysis. It was shown that magnetic resonance imaging (MRI) can identify patients within the time window for thrombolysis (a parts per thousand currency signaEuro parts per thousand 4.5 h from symptom onset) by a mismatch between the acute ischemic lesion visible on diffusion-weighted imaging (DWI) but not visible on fluid-attenuated inversion recovery (FLAIR) imaging. The WAKE-UP trial is an investigator initiated, European, randomized, double-blind, placebo-controlled trial designed to test efficacy and safety of MRI-based thrombolysis with alteplase (tPA) in stroke patients with unknown time of symptom onset, e.g. due to symptom recognition on awakening. A total of 800 patients showing MRI findings of a DWI-FLAIR-mismatch will be randomized to either tPA or placebo. The primary efficacy endpoint will be favourable outcome defined by a modified Rankin scale score 0-1 at day 90. The primary safety outcome measures will be mortality and death or dependency defined by modified Rankin scale score 4-6 at 90 days. If positive the WAKE-UP trial is expected to change clinical practice and to make effective and safe treatment available for a large group of acute stroke patients currently excluded from specific acute treatment.