The purpose of the study was to examine triggers for emergency team activation in hospitals with or without a medical emergency team (MET) system. Materials and Methods: Within a cluster randomized controlled trial examining the effect of introducing a MET system, we recorded the triggers for emergency team activation. We compared the proportion and rate of such triggers in hospitals with or without a MET system and in relation to type of hospital, type of patient ward, and time of day. Results: In control hospitals, the most common trigger for emergency team activation was a decrease in Glasgow Coma Score by 2 or more points (45.6 ), whereas in MET hospitals, it was the fact that staff members were worried or the call occurred despite the lack of a specified reason (39.3 ). In particular, MET hospitals were 35 times more likely to make a call because of staff being worried about the patient (14.1 vs 0.4 , P <.001). Control hospitals were also significantly more likely to call an emergency team because of a deteriorating respiratory (P = .003) or pulse (P <.001) rate, more calls had at least 3 triggers for activation (20.8 vs 10.2 , P = .036), and the average number of triggers per call was significantly higher (P = .013). Nonmetropolitan hospitals were more likely to call an emergency team because of respiratory rate abnormalities (33.6 vs 23.2 , P = .015). Coronary care unit calls were more likely to be triggered by abnormalities in pulse rate and systolic blood pressure, and more calls occurred during the period from 6:00 am to noon. Conclusions: In MET hospitals, more emergency team calls are triggered because staff members are worried about the patient; and fewer calls have multiple triggers. Type of hospital, type of ward, and time of day also affect the nature and frequency of triggers for emergency team activation.