Introduction: The prevalence of vitamin D deficiency has been reported to be very high (up to 50%) in criticallyill children admitted in Paediatric ICU, and vitamin D deficiency has been associated with increased mortality,increased length of Paediatric ICU and hospital stay. But the impact of supplementation of vitamin D in thesechildren on the clinical outcome is not very clear.Study Objectives: Primary Objective: To compare the efficacy of single mega dose vitamin D therapy withstandard daily dose vitamin D therapy in vitamin D deficient critically ill children.Secondary Objective: To assess the prevalence of vitamin D deficiency in critically ill children in a tertiary carecentre.Methodology: A study population of 50 children meeting inclusion criteria were enrolled in our study. All thesubjects admitted to our Paediatric ICU were subjected to a detailed history and examination. Vitamin D levelswere sent at admission and children with vitamin D deficiency (< 20ng/dl) were taken as study subjects andrandomized into two groups. One group received single mega dose vitamin D therapy, and the second groupreceived standard daily dose vitamin D therapy. These children were followed up until their Paediatric ICU stayor death. The impact of supplementation was compared among both the groups in terms of length of PaediatricICU stay and mortality.Results: The mean length of Paediatric ICU stay among children receiving a single mega dose vitamin D therapywas 5.04 days compared to 3.56 days among children receiving standard-dose vitamin D therapy (p-value 0.337),suggesting no significant difference in the length of Paediatric ICU stay among the study subjects of both groups.There was no mortality among these study subjects. The prevalence of vitamin D deficiency in the study groupwas 55.6%.Conclusion: Supplementation of a single mega dose of vitamin D compared to the standard dose of vitamin D hasno advantage on the clinical outcome of Vitamin D deficient critically ill children in terms of length of PaediatricICU stay and mortality.