Stroke thrombolysis benefit decays with time

Pooled results of several trials comparing recombinant tissue plasminogen activator with placebo in ischaemic stroke quantify the profile of benefit and harm for alteplase in broadly selected patients. Generally, alteplase appears to improve the outcome of one in three patients treated between 1 and 3 h from onset and of one in six patients treated in the 3–4·5 h window, but confers no net benefit beyond that time. Benefit may decrease exponentially (according to an accompanying editorial), so if you are a believer then get in there early.
Time to treatment with intravenous alteplase and outcome in stroke: an updated pooled analysis of ECASS, ATLANTIS, NINDS, and EPITHET trials
Lancet. 2010 May 15;375(9727):1695-703