PURPOSE: To prospectively evaluate the implementation of a rapid response team in the form of a medical emergency team (MET) with regard to cardiac arrests and hospital mortality. METHODS: Prospective before-and-after trial of implementation of a MET at the Karolinska University Hospital, Stockholm, Sweden. All adult patients, apart from cardiothoracic, admitted to the hospital were regarded as participants in the study. A control period of 5 years and 203,892 patients preceded the 2-year intervention period of 73,825 patients. MAIN RESULTS: Number of MET calls was 9.3 per 1,000 hospital admissions. Cardiac arrests per 1,000 admissions decreased from 1.12 to 0.83, OR 0.74 (95% CI 0.55-0.98, p = 0.035). Adjusted for age, sex, hospital length of stay, acute/elective admission as well as co-morbidities, MET implementation was associated with a reduction in total hospital mortality by 10%, OR 0.90 (95% CI 0.84-0.97), p = 0.003. Hospital mortality was also reduced for medical patients by 12%, OR 0.88 (95% CI 0.81-0.96, p = 0.002) and for surgical patients not operated upon by 28%, OR 0.72 (95% CI 0.56-0.92, p = 0.008). FOR PATIENTS FULFILLING THE MET CRITERIA: Thirty-day mortality pre-MET was 25% versus 7.9% following MET compared with historical controls. Similarly, 180-day mortality was 37.5% versus 15.8%, respectively. CONCLUSIONS: Implementing the MET team was associated with significant improvement in both cardiac arrest rate and overall adjusted hospital mortality. Significant reductions in hospital mortality for un-operated surgical patients as well as for medical patients were also seen. Thus, introduction of the MET seemed to improve outcome for hospitalized patients.