INTRODUCTION: Growing evidence suggests there is potential harm associated with excess fluid in critically ill children. This study aimed to evaluate the association between percentage fluid overload (%FO) and paediatric intensive care unit (PICU) mortality in children with severe sepsis and septic shock. MATERIALS AND METHODS: Patients with severe sepsis and septic shock admitted to the PICU were identified through discharge codes. Data on clinical characteristics, fluid input and output were collected. %FO was calculated as: (total daily input - total daily output [L]/admission body weight [kg]) × 100. The primary outcome was PICU mortality. Secondary outcomes were 28-day ventilator-free days (VFD), intensive care unit-free days (IFD) and inotrope-free days (InoFD). Multivariate analysis adjusting for presence of comorbidities, Pediatric Index of Mortality (PIM) 2 score and multiorgan dysfunction were used to determine the association between cumulative %FO over 5 days and outcomes. RESULTS: A total of 116 patients were identified, with a mortality rate of 28.4% (33/116). Overall median age was 105.9 (23.1-157.2) months. Cumulative %FO over 5 days was higher in non-survivors compared to survivors (median [interquartile range], 15.1 [6.3-27.1] vs 3.6 [0.7-11.1]%; <0.001). Cumulative %FO was associated with increased mortality (adjusted odds ratio 1.08, 95% confidence interval 1.03-1.13; = 0.001) and decreased VFD, IFD and InoFD (adjusted mean difference -0.37 [-0.53 - -0.21] days, -0.34 [-0.49 - -0.20] days, and -0.31 [-0.48 - -0.14] days, respectively). CONCLUSION Cumulative %FO within the first 5 days of PICU stay was consistently and independently associated with poor clinical outcomes in children with severe sepsis and septic shock. Future studies are needed to test the impact of restrictive fluid strategies in these children.