Abstract: Background: The Rapid Response Call (RRC) is a system designed to escalate care to a specialised team in response to the detection of patient deterioration. To date, there have been few studies which have explored the relationship between time of day of RRC and patient outcome. Objective: To examine the relationship between the time of RRC activations and patient outcome. Methods: All adult inpatients with a RRC in non-critical care wards of a metropolitan Australian hospital in 2012 were retrospectively reviewed. RRCs occurring between 18:00–07:59 were defined as ‘out of hours’. Results: There were 892 RRC during the study period. RRCs out of hours were associated with a higher rate of ICU admissions immediately after the RRC (19.4% VS 12.3%, P<0.001). Patients experiencing an out-of-hours RRC were more likely to have an in-hospital cardiopulmonary arrest (OR=1.7, P<0.04). In-hospital mortality rate was significantly higher for patients with out-of-hours RRCs (35.5% VS 25.0%, P=0.014). After adjusting for confounders out-of-hours RRC were independently associated with increased need for ICU admissions and in-hospital mortality. Conclusion: The diurnal timing of RRCs appears to have significant implications for patient mortality and morbidity, patient outcomes are worse if RRC occurs out of hours. This finding has implications for staffing and resource allocation.