BACKGROUND: Reversibility of the diffusion-weighted imaging (DWI) lesion means that not all of the DWI lesion represents permanently injured tissue. We investigated DWI reversibility and the association with thrombolysis, reperfusion and functional outcome in patients from the WAKE-UP trial (Efficacy and Safety of Magnetic Resonance Imaging–Based Thrombolysis in Wake-Up Stroke). METHODS: In this retrospective analysis of WAKE-UP, a randomized controlled trial (RCT) between September 2012 and June 2017 in Belgium, Denmark, France, Germany, Spain and United Kingdom, a convolutional neural network segmented the DWI lesions (b=1000 s/mm ² ) at baseline and follow-up (24 hours). We calculated absolute and relative DWI reversibility in 2 ways: first, a volumetric (baseline volume−24-hour volume >0) and second, a voxel-based (part of baseline lesion not overlapping with 24-hour lesion) approach. We additionally defined relative voxel-based DWI-reversibility >50% to account for coregistration inaccuracies. We calculated the odds ratio for reversibility according to treatment arm. We analyzed the association of reversibility with excellent functional outcome (modified Rankin Scale score of 0–1), in a multivariable model. RESULTS: In 363 patients, the median DWI volume was 3 (1–10) mL at baseline and 6 (2–20) mL at follow-up. Volumetric DWI reversibility was present in 19% (69/363) with a median absolute reversible volume of 1 mL (0–2) or 28% (14–50) relatively. Voxel-based DWI reversibility was present in 358/363 (99%) with a median absolute volume of 1 mL (0–2), or 22% (9–38) relatively. In 18% of the patients (67/363), relative voxel-based DWI reversibility >50% was present. Volumetric DWI reversibility and relative voxel-based DWI reversibility >50% was more frequent in patients treated with alteplase versus placebo (OR, 1.86 [95% CI, 1.09–3.17] and OR, 2.03 [95% CI, 1.18–3.50], respectively). Relative voxel-based DWI reversibility >50% was associated with excellent functional outcome (OR, 2.30 [95% CI, 1.17–4.51]). CONCLUSIONS: Small absolute volumes of DWI reversibility were present in a large proportion of randomized patients in the WAKE-UP trial. Reversibility was more often present after thrombolysis.