BACKGROUND: Most studies of Rapid-Response Teams (RRTs) assess their effect on outcomes of all hospitalised patients. Little information exists on RRT activation patterns or why RRT calls are needed. Triage error may necessitate RRT review of ward patients shortly after hospital admission. RRT diurnal activation rates may reflect the likely frequency of caregiver visits. OBJECTIVES: To study the timing of RRT calls in relation to time of day and day of week, and their frequency and outcomes in relation to days after hospital admission. METHODS: We prospectively studied RRT calls over 1 month in seven hospitals during 2009, collecting data on patient age, sex, admitting unit, admission source, limitations of medical therapy (LOMTs), and admission and discharge dates. We assessed the timing of RRT calls in relation to hospital admission and circadian variation; and differences in characteristics and outcomes of calls occurring early (Days 0 and 1) versus late (after Day 7) after hospital admission. RESULTS: There were 652 RRT calls for 518 patients. Calls were more likely on Mondays (P=0.018) and during work hours (P<0.0001) but less likely on weekends (P=0.003) or overnight (P<0.001). There were 177 early calls (27.1%) and 198 late calls (30.4%). Early calls involved younger patients (median ages, 67.5 years [early calls] v 73 years [late calls]; P= 0.01), fewer LOMTs (P=0.029), and lower in hospital mortality (12.8% [early calls] v 32.3% [late calls]; P<0.0001). The mortality difference remained in patients without LOMTs (5.6% [early calls] v 19.6% [late calls]; P=0.003). CONCLUSIONS: About one-quarter of RRT calls occurred shortly after hospital admission, and were more common when caregivers were around. Early calls may partially reflect suboptimal triage, though the associated mortality appeared low. Late calls may reflect suboptimal end-of-life care planning, and the associated mortality was high. There is a need to further assess the epidemiology of RRT calls at different phases of the hospital stay.