Objective: We tested the hypothesis that responses to physiologic deterioration in hospital ward patients delayed by more than 15 minutes are associated with increased mortality. Design, Setting, and Participants: We used data from a 23-hospital cluster randomized trial (January 2004 to December 2004) of implementation of rapid response teams (intervention) versus standard practice with conventional cardiac arrest team-based responses to emergencies (control). We examined emergency calls in all hospitals. In intervention hospitals, we also examined such calls in the period before, during the introduction, and after the full implementation of a rapid response system. We studied the statistical association between such delayed calls and mortality. Main Outcomes and Measures: Hospital outcomes (mortality, unplanned ICU admissions, and cardiac arrests). Results: There were 3,135 emergency team calls in all hospitals. Overall, almost one third of such calls were delayed. In intervention hospitals, the proportion of delayed calls was similar before and after implementation of rapid response teams. Compared with control hospitals, in intervention hospitals, there was a significant decrease in the proportion of delayed calls during both the introduction (27.3 vs 34.3 weekly rate; incidence rate ratio, 0.84; p = 0.001) and the full implementation period (29.0 vs 34.5 weekly rate; incidence rate ratio, 0.84; p = 0.023). Delayed calls more likely occurred at night, in high dependence or coronary care units, in patients older than 75 years, in those with a decrease in Glasgow Coma Scale, or in those with hypotension as the reason for the call. Finally, in all hospitals, delayed calls were associated with an increased risk of unplanned ICU admissions (adjusted odds ratio = 1.56; 95 CI, 1.23-2.04; p = 0.001) and death (adjusted odds ratio = 1.79; 95 CI, 1.43-2.27; p <0.001). Conclusions: Among ward patients, emergency team activation in response to acute deterioration triggered more than 15 minutes after detection and documentation of instability is independently associated with an increased risk of ICU admission and death.