This is an open randomised prospective pilot exploratory study of corticosteroid replacement therapy in three centres. Adrenal function measurements will be assessed on entry to the study. To investigate the inflammatory profile and the impact of corticosteroid replacement, blood will be taken for cytokine and coagulation protein analysis. This study will provide the pilot data necessary for the design of a definitive trial of corticosteroid replacement therapy with the identification of variables likely to improve our ability to stratify patients for intervention and the mechanistic characterisation of the modulatory effects of steroids on inflammation in children with severe sepsis. Enrolment will be undertaken in two stages (see flowsheet diagrams in protocol). Forty five eligible children will be randomly allocated to steroid replacement therapy for 2 days (n=30) or intensive investigation without intervention (n=15) in a 2:1 randomisation (stage 1); 45 subjects (stage 2) will then be randomly allocated to steroid replacement therapy for 5 days (n=30) or intensive investigation without intervention (n=15). Randomisation will the undertaken in accordance with a computer−generated list and will be stratified by age (<1 years; 1 year or more). Progression from stage 1 to stage 2 will follow an interim analysis by a Trial Monitoring Group to ensure safety. This escalating approach will provide safety data, information on length of therapy and an assessment of possible clinical and laboratory endpoints in addition to mortality, reducing the potential for adverse events in the pilot phase while providing data relevant to this population. A large excess of serious adverse events in stage 1 will result in study termination. After careful consideration by the investigators and during the peer review process, placebo will not be used in this study, which will inform a future large phase 3 randomised controlled trial.