Background: Previous studies have suggested that desmoteplase, a novel plasminogen activator, has clinical benefit when given 3-9 h after the onset of the symptoms of stroke in patients with presumptive tissue at risk that is identified by magnetic resonance perfusion imaging (PI) and diffusion-weighted imaging (DWI). Methods: In this randomised, placebo-controlled, double-blind, dose-ranging study, patients with acute ischaemic stroke and tissue at risk seen on either MRI or CT imaging were randomly assigned (1:1:1) to 90 μg/kg desmoteplase, 125 μg/kg desmoteplase, or placebo within 3-9 h after the onset of symptoms of stroke. The primary endpoint was clinical response rates at day 90, defined as a composite of improvement in National Institutes of Health stroke scale (NIHSS) score of 8 points or more or an NIHSS score of 1 point or less, a modified Rankin scale score of 0-2 points, and a Barthel index of 75-100. Secondary endpoints included change in lesion volume between baseline and day 30, rates of symptomatic intracranial haemorrhage, and mortality rates. Analysis was by intention to treat. This study is registered with ClinicalTrials.gov, NCT00111852. Findings: Between June, 2005, and March, 2007, 193 patients were randomised, and 186 patients received treatment: 57 received 90 μg/kg desmoteplase; 66 received 125 μg/kg desmoteplase; and 63 received placebo. 158 patients completed the study. The median baseline NIHSS score was 9 (IQR 6-14) points, and 30% (53 of 179) of the patients had a visible occlusion of a vessel at presentation. The core lesion and the mismatch volumes were small (median volumes were 10·6 cm3 and 52·5 cm3, respectively). The clinical response rates at day 90 were 47% (27 of 57) for 90 μg/kg desmoteplase, 36% (24 of 66) for 125 μg/kg desmoteplase, and 46% (29 of 63) for placebo. The median changes in lesion volume were: 90 μg/kg desmoteplase 14·0% (0·5 cm3); 125 μg/kg desmoteplase 10·8% (0·3 cm3); placebo -10·0% (-0·9 cm3). The rates of symptomatic intracranial haemorrhage were 3·5% (2 of 57) for 90 μg/kg desmoteplase, 4·5% (3 of 66) for 125 μg/kg desmoteplase, and 0% for placebo. The overall mortality rate was 11% (5% [3 of 57] for 90 μg/kg desmoteplase; 21% [14 of 66] for 125 μg/kg desmoteplase; and 6% [4 of 63] for placebo). Interpretation: The DIAS-2 study did not show a benefit of desmoteplase given 3-9 h after the onset of stroke. The high response rate in the placebo group could be explained by the mild strokes recorded (low baseline NIHSS scores, small core lesions, and small mismatch volumes that were associated with no vessel occlusions), which possibly reduced the potential to detect any effect of desmoteplase. Funding: PAION Deutschland GmbH; Forest Laboratories. © 2009 Elsevier Ltd. All rights reserved.