Background and Purpose-The DIAS-2 study was the only large, randomized, intravenous, thrombolytic trial that selected patients based on the presence of ischemic penumbra. However, DIAS-2 did not confirm the positive findings of the smaller DEDAS and DIAS trials, which also used penumbral selection. Therefore, a reevaluation of the penumbra selection strategy is warranted. Methods-In post hoc analyses we assessed the relationships of magnetic resonance imaging-measured lesion volumes with clinical measures in DIAS-2, and the relationships of the presence and size of the diffusion-perfusion mismatch with the clinical effect of desmoteplase in DIAS-2 and in pooled data from DIAS, DEDAS, and DIAS-2. Results-In DIAS-2, lesion volumes correlated with National Institutes of Health Stroke Scale (NIHSS) at both baseline and final time points (PϽ0.0001), and lesion growth was inversely related to good clinical outcome (Pϭ0.004). In the pooled analysis, desmoteplase was associated with 47% clinical response rate (nϭ143) vs 34% in placebo (nϭ73; Pϭ0.08). For both the pooled sample and for DIAS-2, increasing the minimum baseline mismatch volume (MMV) for inclusion increased the desmoteplase effect size. The odds ratio for good clinical response between desmoteplase and placebo treatment was 2.83 (95% confidence interval, 1.16 -6.94; Pϭ0.023) for MMV Ͼ60 mL. Increasing the minimum NIHSS score for inclusion did not affect treatment effect size. Key Words: acute cerebral infarction Ⅲ desmoteplase Ⅲ diffusion-weighted imaging Ⅲ magnetic resonance imaging Ⅲ mismatch Ⅲ perfusion Ⅲ stroke Ⅲ thrombolysis R eperfusion of the ischemic penumbra (ie, at-risk but salvageable ischemic brain) is the goal of thrombolytic stroke therapy. Up to 4.5 hours from symptom onset, thrombolytic treatment efficacy is demonstrable in patients with clinical ischemic stroke syndrome. 1,2 Imaging confirmation of the ischemic penumbra has not been required in this treatment period because virtually all patients are assumed to have salvageable tissue in the early period after symptom onset. However, spontaneous recanalization and reperfusion as well as progression of penumbral tissue to infarction occur at variable rates and cannot be predicted by clinical features. After 4.5 hours, many patients may no longer have the target ischemic penumbra that would justify thrombolysis, and their inclusion into clinical trials would confound the detection of therapeutic benefit. A rational approach to expanding the indication for thrombolytic stroke therapy would be selection of only those patients who have evidence of ischemic penumbra. Magnetic resonance imaging (MRI) diffusion-weighted imaging and perfusion-weighted (PWI-DWI) mismatch permits a clinical trial strategy to select patients with ischemic penumbra and to monitor drug effects on the probability of reperfusion and infarct expansion. [3][4][5] On MRI, the ischemic penumbra is approximated by a region of mismatch between a larger perfusion defect relative to a smaller region of acute ischemic DWI change, an...