Background A large proportion of deaths among patients with myocardial infarction occurs within the first 24 hours after presentation. It is not clear whether this phenomenon is also true of patients without ST-segment elevation who may or may not have infarction at the time of presentation. Thrombin activity may also be greatest during the first 24 hours after plaque rupture. Accordingly, this study was designed to examine the pattern of early ischemic events among patients with acute coronary syndromes and to determine whether the direct thrombin inhibitor desirudin (r-hirudin) would be most effective during this period. Methods and Results Among the 11,142 patients enrolled in GUSTO-II, death or (re)infarction occurred within 24 hours in 210 patients (1.7%), representing 19% of the 1135 deaths that had occurred by 30 days. Death or (re)infarction occurred within 24 hours in 113 patients (2.7%) with ST-segment elevation and in 97 patients without ST-segment elevation (1.2%, P < .001), representing 26% and 14% of the 30-day event rates, respectively, for the 2 enrollment strata. Among patients with ST-segment elevation, most of these events were deaths, whereas among patients without ST-segment elevation, most events were (re)infarctions. Death or (re)infarction by 24 hours occurred in 80 (1.3%) patients treated with desirudin and 130 (2.1%) patients treated with heparin (P = .01). This finding predominantly consisted of prevention of death among patients with ST-segment elevation and of (re)infarction among patients without ST-segment elevation. Conclusions These findings have important implications for early triage of patients with acute coronary syndromes and for the development of new therapies directed at stabilizing the unstable atherosclerotic plaque. (Am Heart J 1999;137:12-23.)