BACKGROUND In hospital cardiac arrests (CA) treated with cardio-pulmonary resuscitation (CPR) outside of the intensive care unit (ICU) have poor outcomes. Most are preceded by deranged vital signs. There are, however, limited studies assessing antecedents to CAs inside the ICU. OBJECTIVE To study the antecedents to, and characteristics of CAs in ICU. METHODS We prospectively identified CA cases that occurred inside our ICU between January 2010 and July 2012. Controls were obtained by sequentially matching ICU patients based on APACHE III diagnosis, APACHE III score, age, gender and length of stay in ICU. RESULTS Thirty-six patients had a CA during the study period (6.28/1000 admissions). In the 12h prior to CA, index patients had higher maximum (22 breaths/min vs. 18 breaths/min, p=0.001) and minimum respiratory rates (16 breaths/min vs. 12 breaths/min, p=0.031), a lower median mean arterial pressure (65 mmHg vs. 70 mmHg, p=0.029) and systolic blood pressure (97 mmHg vs. 106 mmHg, p=0.033), a higher central venous pressure (14 cm H2O vs. 11 cm H2O, p=0.008) and a lower bicarbonate level (20.5 mmol vs. 26 mmol, p=0.018) compared to controls. CA patients also had a higher maximum dose of noradrenaline (norepinephrine) (17.5 mcg/min vs. 8.0 mcg/min, p=0.052) but there was no difference in any other levels of intensive care support. Two-thirds of CA's occurred within the first 48 h of ICU admission. The initial monitored rhythm was non-shock responsive (pulseless electrical activity, bradycardia or asystole) in 26/36 (72%). Return of spontaneous circulation was achieved in 29/36 (80.6%) patients, with 16/36 (44.4%) surviving to hospital discharge. CONCLUSIONS In the period leading up to the CA inside ICU, there were signs of physiological instability and the need for higher doses of noradrenaline. Return of spontaneous circulation was achieved in 80%. However, in-hospital mortality was greater than 50%.