To determine whether the introduction of an intensive care unit-based medical emergency team, responding to hospital-wide preset criteria of physiologic instability, would decrease the rate of predefined adverse outcomes in patients having major surgery. Prospective, controlled before-and-after trial. University-affiliated hospital. Consecutive patients admitted to hospital for major surgery during a 4-month control phase and during a 4-month intervention phase. Introduction of a hospital-wide intensive care unit-based medical emergency team to evaluate and treat in-patients deemed at risk of developing an adverse outcome by nursing, paramedical, and/or medical staff. We measured incidence of serious adverse events, mortality after major surgery, and mean duration of hospital stay. There were 1,369 operations in 1,116 patients during the control period and 1,313 in 1,067 patients during the medical emergency team intervention period. In the control period, there were 336 adverse outcomes in 190 patients (301 outcomes/1,000 surgical admissions), which decreased to 136 in 105 patients (127 outcomes/1,000 surgical admissions) during the intervention period (relative risk reduction, 57.8%; p <.0001). These changes were due to significant decreases in the number of cases of respiratory failure (relative risk reduction, 79.1%; p <.0001), stroke (relative risk reduction, 78.2%; p =.0026), severe sepsis (relative risk reduction, 74.3%; p =.0044), and acute renal failure requiring renal replacement therapy (relative risk reduction, 88.5%; p <.0001). Emergency intensive care unit admissions were also reduced (relative risk reduction, 44.4%; p =.001). The introduction of the medical emergency team was also associated with a significant decrease in the number of postoperative deaths (relative risk reduction, 36.6%; p =.0178). Duration of hospital stay after major surgery decreased from a mean of 23.8 days to 19.8 days (p =.0092). The introduction of an intensive care unit-based medical emergency team in a teaching hospital was associated with a reduced incidence of postoperative adverse outcomes, postoperative mortality rate, and mean duration of hospital stay.