We randomly assigned 52 patients to alteplase and 49 patients to placebo. Mean age was 71.6 years, and median score on the National Institutes of Health stroke scale was 13. 85 of 99 (86%) patients had mismatch of PWI and DWI. The geometric mean infarct growth (exponential of the mean log of relative growth) was 1.24 with alteplase and 1.78 with placebo (ratio 0.69, 95% CI 0.38-1.28; Student's t test p=0.239); the median relative infarct growth was 1.18 with alteplase and 1.79 with placebo (ratio 0.66, 0.36-0.92; Wilcoxon's test p=0.054). Reperfusion was more common with alteplase than with placebo and was associated with less infarct growth (p=0.001), better neurological outcome (p<0.0001), and better functional outcome (p=0.010) than was no reperfusion. Alteplase was non-significantly associated with lower infarct growth and significantly associated with increased reperfusion in patients who had mismatch. Because reperfusion was associated with improved clinical outcomes, phase III trials beyond 3 h after treatment are warranted.