The presence of congestive heart failure (CHF) has been associated with treatment disparities and worse outcomes in patients with ST-segment elevation myocardial infarction, but the incidence and effect of CHF in patients with non-ST-segment elevation acute coronary syndromes (NSTE ACSs) has not been well characterized. We evaluated 45,744 patients with NSTE ACS (positive cardiac markers and/or ischemic ST-segment changes) who were treated at 424 hospitals in the CRUSADE Quality Improvement Initiative between March 2000 and March 2003. Treatment patterns and in-hospital outcomes in patients with signs of CHF on presentation and those who developed in-hospital CHF were compared with those in patients without CHF. In total, 10,398 patients (22.7%) had signs of CHF on presentation, and 1,664 patients (3.6%) later developed in-hospital CHF. Compared with patients without CHF, early (<24 hours from presentation) medications and invasive cardiac procedures were used less often in patients with signs of CHF on presentation. Likewise, patients with in-hospital CHF were less likely than those without CHF to receive acute antiplatelet agents and undergo cardiac catheterization but more likely to receive acute beta blockers, angiotensin-converting enzyme inhibitors, and heparin and to undergo coronary artery bypass grafting. Adjusted mortality was higher in patients with signs of CHF on presentation (odds ratio 2.64, 95% confidence interval 2.31 to 3.01) and those with in-hospital CHF (odds ratio 4.93, 95% confidence interval 4.05 to 5.99) than in patients without CHF. In conclusion, CHF occurs frequently in patients with NSTE ACS but is associated with less aggressive treatment and a higher risk of mortality. Further study is needed to determine the causes of these treatment differences and the optimal therapeutic approach for patients with NSTE ACS and concomitant CHF.