There are limited data regarding the incidence and clinical significance of congestive heart failure (CHF) in patients with non-ST segment elevation acute coronary syndromes (ACS). The objectives of this study were to examine the incidence, predictors, and clinical outcomes in patients with ACS without ST elevation who develop CHF. We studied patients with unstable angina or non-ST segment elevation myocardial infarction (NSTEMI) randomized to hirudin or unfractionated heparin in the Organisation to Assess Strategies for Ischemic Syndromes (OASIS-2) trial. The diagnosis of CHF was based on a combination of clinical and radiographic features. Patients were followed for 6 months. Of 10 141 randomized patients, 501 (4.9%) developed CHF within the first week and 643 (6.3%) during 6 months of followup. Independent predictors for the development of CHF were older age, female sex, diabetes, prior MI, prior CHF, and NSTEMI at presentation. Compared with patients who did not develop CHF, patients who developed CHF were at increased risk of death (odds ratio (OR) 3.4, 95% CI 2.7–4.3), new MI (OR 2.8, 95% CI 2.2–3.6), and the need for intra-aortic balloon pump insertion (OR 5.4, 95% CI 3.5–8.4) at 7 days and 6 months. There was no increase in use of cardiac catheterization (OR 0.8, 95% CI 0.7–1.0) or revascularization (OR 0.9, 95% CI 0.7–1.1) in patients who developed CHF. CHF is a common complication in patients presenting with non-ST segment elevation ACS and is strongly associated with adverse clinical outcomes including new MI and death. Despite this worse prognosis, patients with ACS developing CHF are less likely to be referred for invasive management.Key words: unstable angina, acute coronary syndrome, congestive heart failure, prognosis.