BACKGROUND: Although randomized controlled trials support the use of intensive medical and invasive therapies for non-ST segment elevation acute coronary syndromes (NSTE-ACS), major bleeding is a serious treatment complication. We sought to determine the temporal trend of in-hospital major bleeding among patients with NSTE-ACS, in relation to the evolving management pattern. METHODS: We identified 14 111 NSTE-ACS patients enrolled in 4 successive, prospective, multicenter registries (ACS I, 1999-2001; ACS II, 2002-2003; GRACE, 2004-2007; and CANRACE, 2008) in Canada between 1999 and 2008. We collected data on patient characteristics, use of cardiac medications and procedures on standardized case report forms. In all registries, major bleeding was defined a priori as life threatening or fatal bleeding, bleeding requiring transfusion of >/=2 U of packed red cells, or resulting in an absolute decrease in hemoglobin of >30g/L. RESULTS: A total of 14 111 patients had a final diagnosis of NSTE-ACS and were included in this study (3294 in the ACS-I registry, 1956 in the ACS-II registry, 7543 in GRACE, and 1318 in CANRACE). Over time, there was a substantial increase in the use of dual anti-platelet (aspirin and thienopyridine) therapy (P for trend <.001), and in rates of in-hospital cardiac catheterization and percutaneous coronary intervention (both Ps for trend <.001). Overall, major bleeding was relatively infrequent (1.7%). There was no significant increase in the unadjusted rates of major bleeding over time (P for trend = .19). In multivariable analysis adjusting for GRACE risk score and intensive treatment, enrolment period was not an independent predictor of bleeding (P for trend = .98). There was no interaction between the enrolment period and the use of intensive medical and invasive management. CONCLUSION: Despite more widespread use of dual anti-platelet therapies and invasive cardiac procedures in the management of NSTE-ACS, the rate of major bleeding remains relatively low and has not increased significantly over time. Our findings suggest that physicians selectively target treatment for their patients, and these evidence-based therapies can be safely administered to ACS patients in clinical practice.