Many deaths, cardiac arrests, and unplanned intensive care unit (ICU) admissions are preceded by failure to recognize deteriorating vital signs1,2 or trigger intervention.3,4 Furthermore, survival following cardiovascular collapse is low, whereas early response likely improves survival.5,6 As such, in-hospital medical emergency teams (MET) were proposed. Physiologic triggers (see below) lead to their activation. The first multi-site prospective MET trial. Cluster randomized-control trial of 23 Australian hospitals. Inclusion criteria were public hospitals with > 20,000 annual admissions, an ICU and emergency department, and no pre-existing MET. Exclusion criteria were patients under14 yr, or not formally admitted. MET activation criteria were: 1) airway: if threatened; 2) breathing: respiratory arrests, respiratory rate 36 breaths·min-1; 3) circulation: cardiac arrests, pulse rate 140, systolic blood pressure 2 Glasgow coma scale points, repeated or extended seizures; 5) other: patients not meeting above criteria but ward staff seriously worried. Cardiac arrests were defined by absence of a palpable pulse; unplanned ICU admission as unscheduled transfer from the ward to the ICU; and unexpected deaths were those without a pre-existing not-for-resuscitation (NFR) order. Hospitals were randomized to no change in current functioning (maintenance of an arrest team) or initiation of MET. This followed four months of instruction for clinical staff in MET hospitals, before a six-month study period. Standardized MET instruction focused on reinforcing MET calling-criteria, the importance of identifying at-risk patients and the need to promptly initiate the MET. Composite outcome of the incidence (events divided by eligible patients admitted to hospital) for 1) cardiac arrests without a NFR; 2) unplanned ICU admission; and 3) unexpected deaths on general wards. MET initiation was associated with significantly more calls to resuscitation (cardiac arrest or medical emergency) teams (3.1vs 8.7 per 1,000 admissionsP = 0.0001). Only 30% of unplanned ICU admissions were preceded by MET activation, but this was significantly more than 9% for control hospitals (P = 0.009). There was no significant difference in primary outcome (5.86 controlvs 5.31 MET per 1,000 admissions,P = 0.640). There also were no differences in the secondary outcomes: cardiac arrest (1.64vs 1.31 per 1,000 admissionsP = 0.736); unplanned ICU admission (4.68 vs 4.19 per 1,000 admissionsP = 0.599) or unexpected death (1.18vs 1.06 per 100 admissionsP = 0.752). There was a decrease in the rate of cardiac arrests (P = 0.03) and unexpected deaths (P = 0.01) over six months for both the MET and control hospitals. The MET system greatly increased the frequency of emergency team calling but did not decrease cardiac arrests, unplanned ICU admissions or unexpected death.