OBJECTIVE: To determine the effect on cardiac arrests and overall hospital mortality of an intensive care-based medical emergency team. DESIGN AND SETTING: Prospective before-and-after trial in a tertiary referral hospital. PATIENTS: Consecutive patients admitted to hospital during a 4-month "before" period (May-August 1999) (n = 21 090) and a 4-month intervention period (November 2000 -February 2001) (n = 20 921). MAIN OUTCOME MEASURES: Number of cardiac arrests, number of patients dying after cardiac arrest, number of postcardiac-arrest bed-days and overall number of in-hospital deaths. RESULTS: There were 63 cardiac arrests in the "before" period and 22 in the intervention period (relative risk reduction, RRR: 65%; P < 0.001). Thirty-seven deaths were attributed to cardiac arrests in the "before" period and 16 in the intervention period (RRR: 56%; P = 0.005). Survivors of cardiac arrest in the "before" period required 163 ICU bed-days versus 33 in the intervention period (RRR: 80%; P < 0.001), and 1353 hospital bed-days versus 159 in the intervention period (RRR: 88%; P < 0.001). There were 302 deaths in the "before" period and 222 in the intervention period (RRR: 26%; P = 0.004). CONCLUSIONS: The incidence of in-hospital cardiac arrest and death following cardiac arrest, bed occupancy related to cardiac arrest, and overall in-hospital mortality decreased after introducing an intensive care-based medical emergency team.