Outcome after ST segment elevation myocardial infarction (STEMI), has improved but patients with high Killip class still have a poor prognosis, and those ≥II need a closer monitoring in a specialized cardiac care unit.We aimed to determine the predictors of Killip class in a group of patients admitted for acute STEMI.Non-interventional registry in a Cardiac Intensive Care Unit. Patients were consecutively included from January 2010 to April 2015, and multivariate analysis was performed to determine independent predictors of high Killip Class.We included 1111 patients, mean age was 64.0±14.0years and 258 (23.2%) were female. Primary percutaneous coronary intervention was performed in 991 (89.2%), and 120 (10.8%) only received thrombolysis as acute reperfusion therapy. A total of 230 (20.7%) were in class II or higher. The independent predictors of Killip≥II were (odds ratio [95% confidence interval]): older age (2.1 [1.4–3.0]), female sex (1.6 [1.1–2.2]), diabetes (1.4 [1.0–2.1]), prior heart failure (3.2 [1.4–7.2]), chronic kidney disease (2.0 [1.1–3.6]), anaemia (3.0 [2.0–4.5]), multivessel disease (1.6 [1.1–2.2]), anterior location (2.4 [1.8–3.4]), time of evolution>2h (1.6 [1.1–2.4]), and TIMI flow-grade<3 (1.8 [1.2–2.7]). In-hospital mortality increased with Killip class (I 1.5%, II 3.7%, III 16.7%, IV 36.7%).In patients with STEMI Killip class can be predicted with variables available when primary percutaneous coronary intervention is performed and is strongly associated with in-hospital prognosis.