Background and objective: The objective of this project is to investigate the factors predicting mortality and mean length of stay in patients diagnosed with unstable angina (UA) during admission to the Intensive Care Unit or Critical Care Unit (ICU/CCU). Patients and method: A retrospective cohort study including all the UA patients listed in the Spanish ARIAM register. The study period comprised from June, 1996 to December, 2003. The follow-up period is limited to the stay in the ICU/CCU. One univariate analysis was performed between deceased and live patients; and another between prolonged and non-prolonged stay patients. Three multivariate analyses were also performed; one to evaluate the factors related to mortality, another to evaluate the variables associated to percutaneous coronary intervention (PCI) and another to evaluate the factors associated to the prolonged mean stay in ICU/CCU. Results: 14,096 patients with UA were included in the study. The UA mortality rate during ICU/CCU admission was 1.1%. Mortality was associated to Killip classification, age, the need for CPR, development of cardiogenic shock, development of arrhythmia (such as VF, sinus tachycardia or high-degree atrioventricular block) and diabetes; whereas patients who smoke were associated to a lower mortality rate. PCI was only performed in 1,226 patients (8.9%), increasing over the years. The PCI-predicting variables were: age, being referred from another hospital, smoking, presenting prior acute myocardial infarction (AMI), complications consisting of cardiogenic shock or high-degree atrioventricular block and being treated with oral beta blockers. The mean length of stay in ICU/CCU was 3.15 (18.65) days (median, 2 days), depending on age, a coronariography having previously been performed, the Killip classification, having required coronariography and PCI or echocardiography or mechanical ventilation, and presenting complications such as angina that is difficult to control, arrhythmia, right ventricular failure or death. Conclusions: The factors are associated to mortality were; greater age, diabetes, Killip classification, arrhythmia, cardiogenic shock and the need for CPR, whereas smoking is associated to a lower mortality rate. The patients on whom PCI was performed represent a less severe population. Management has changed over the years, with an increase in PCI. A prolonged mean length of stay is associated to the appearance of arrhythmia, right or left heart failure, angina that is difficult to control, age and PCI