Bleeding is a complication of current therapies for acute coronary syndrome (ACS). No studies have examined the effect of bleeding events on clinical outcomes. We analyzed pooled data from 4 multicenter, randomized clinical trials of patients who had ACS (n = 26,452) to determine an association between bleeding severity as measured by the GUSTO scale and 30-day and 6-month mortality rates using Cox proportional hazards modeling that incorporated bleeding as a time-dependent covariate. The analysis was repeated to examine procedure- and non-procedure-related bleeding and after censoring at the time of coronary artery bypass grafting. Of all the patients included, 27.6% had > or =1 bleeding episode. Patients who bled were older and sicker at presentation than were those who did not bleed. Unadjusted rates of 30-day and 6-month mortality increased as bleeding severity increased. There were stepwise increases in the adjusted hazards of 30-day mortality (mild bleeding, hazard ratio [HR] 1.6, 95% confidence interval [CI] 1.3 to 1.9; moderate bleeding, HR 2.7, 95% CI l 2.3 to 3.4; severe bleeding, HR 10.6, 95% CI 8.3 to 13.6) and 6-month mortality (mild bleeding, HR 1.4, 95% CI 1.2 to 1.6; moderate bleeding, HR 2.1, 95% CI 1.8 to 2.4; severe bleeding, HR 7.5, 95% CI 6.1 to 9.3) as bleeding severity increased. Results were consistent after censoring for coronary artery bypass grafting and for procedure- and non-procedure-related bleeds. In conclusion, the GUSTO bleeding classification identifies patients who are at risk for short- and long-term adverse events. Therapies that minimize bleeding risk and maintain an anticoagulant effect may improve outcomes among patients who have ACS.