The treatment involves intravenous injection of the clot-dissolving enzyme t-PA--the same enzyme used to treat clots in the heart--into patients within three hours after the onset of stroke symptoms.

"This is the first unequivocal evidence of an effective treatment of stroke," said neuroscientist Zach W. Hall, director of the National Institute of Neurological Disorders and Stroke, which sponsored the study. The results "signal the beginning of a new era in which stroke will be recognized as an acute illness that can be treated."

If too much drug is given or treatment is delayed too long, the drug can cause hemorrhaging into the brain. In fact, 6.4% of the patients who received t-PA in the trial suffered from intracranial bleeding, compared to only 0.6% of those who received a placebo. But that bleeding did not affect mortality: Only 17% of those who received t-PA died within 36 hours, compared to 21% of those who received a placebo.