Non-ST-elevation myocardial infarction (NSTEMI) and unstable angina (UA) resulted in different degrees of damage to the heart muscle, and yet, when factors related to in-hospital outcomes were examined, these two subsets were often lumped together as non-STelevation acute coronary syndrome. Therefore, we investigated predictors of in-hospital heart failure (HF) in UA and NSTEMI separately. Factors related to HF (Killip > or = 2) were analyzed for NSTEMI and UA in a Thai Acute Coronary Syndrome (ACS) registry conducted in 17 institutions between 2002 and 2005. The registry comprised of 9373 single admissions age 65.1 +/- 12.3 years, 40.2% women, and 45.1% with HF. There were 3548 NSTEMI and 1989 UA with HF prevalence of 56.2% and 27.4%, respectively. Heart failure patients were older, more were women, sicker (as shown by more of those with shock, postcardiac arrest, and breathless on admission), more with diabetes mellitus (DM), received less intervention and medication, and showed higher total death (19.3% vs 5.3% for NSTEMI with and without HF; and correspondingly, 5.9% and 1.9% for UA). Independent predictors (at presentation) for the development of HF following NSTEMI or UA were age (not sex), breathlessness, and less prevalence of chest pain. However, shock and DM were risks only for NSTEMI but not UA. Heart failure was found to be a factor for in-hospital death for NSTEMI only, with odds ratio of 2.84 (confidence interval 2.11-3.82) and 3.23 (2.25-4.64) for total and cardiac deaths, respectively. Non-ST-elevation myocardial infarction and UA showed substantial differences in factors related to predictors for in-hospital outcome such that these should be examined separately.