Background:In critical illness, there also is evidence of rapid falls in circulating 25(OH)D concentrations. So, we aim to assess the prevalence of VDD in a pediatric ICU (PICU) patients and to determine whether there is any relationship between VDD and illness severity and mortality. Patients and methods:this is a case control study that included 40 children :20 patient admitted to pediatric intensive care unit in Qena university at time of the study(from January 2018 till December 2018) compared to 20 apparently healthy well-matched children regarding age and sex to be compared as controls.All patients subjected to: Full history,detailed physical examination,diagnostic modalities as: laboratory investigation and severity of illness was assessed with the pediatric risk of mortality III Result: Our study included 40 child with the age of patientsrangedfrom5 monthsto12years withmean +/- SD 3.7 ±3 year. Male patients represented70%.In our study, the prevalence of vitamin D deficiency among critically ill children was 70 %we did not findsignificant correlation between 25(OH) vitamin D and pediatric risk of mortality III (PRISM III) Conclusion:Vitamin D Deficiency (VDD) prevalence was reported in 70% of the critically ill patients, and it was observed to be related to multiple organ dysfunctions and rapid clinicaldeterioration.