BACKGROUND AND PURPOSE: Two phase 2 studies of alteplase in acute ischemic stroke 3 to 6 hours after onset, Echoplanar Imaging Thrombolytic Evaluation Trial (EPITHET; a randomized, controlled, double-blinded trial), and Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution Study (DEFUSE; open-label, treatment only) using MR imaging-based outcomes have been conducted. We have pooled individual patient data from these to assess the response to alteplase. The primary hypothesis was that alteplase would significantly attenuate infarct growth compared with placebo in mismatch-selected patients using coregistration techniques. METHODS: The EPITHET-DEFUSE study datasets were pooled while retaining the original inclusion and exclusion criteria. Significant hypoperfusion was defined as a Tmax delay >6 seconds), and coregistration techniques were used to define MR diffusion-weighted imaging/perfusion-weighted imaging mismatch. Neuroimaging, parameters including reperfusion, recanalization, symptomatic intracerebral hemorrhage, and clinical outcomes were assessed. Alteplase and placebo groups were compared for the primary outcome of infarct growth as well for secondary outcome measures. RESULTS: From 165 patients with adequate MR scans in the EPITHET-DEFUSE pooled data, 121 patients (73.3 ) were found to have mismatch. For the primary outcome analysis, 60 patients received alteplase and 41 placebo. Mismatch patients receiving alteplase had significantly attenuated infarct growth compared with placebo (P=0.025). The reperfusion rate was also increased (62.7 vs 31.7 ; P=0.003). Mortality and clinical outcomes were not different between groups. CONCLUSIONS: The data provide further evidence that alteplase significantly attenuates infarct growth and increases reperfusion compared with placebo in the 3- to 6- hour time window in patients selected based on MR penumbral imaging.