Background: Phosphorus is very important for normal cellular structure (cell membrane and nucleic acids). Phosphorus has an important role in the cell metabolism, ATP production, and homeostasis. Objectives: The aim of the work was to determine the prevalence and some of risk factors of hypophosphatemia in sick children, and to evaluate the prognostic effect of serum phosphorous level on the outcome in terms of morbidity and mortality in critically ill children during their stay in the pediatric intensive care unit. Patients and methods: This prospective cohort study included a total of 50 critically ill children, attending at Department of Biochemistry and Pediatric Intensive Care Unit (PICU), Department of Pediatrics, Zagazig University Hospitals. This study was conducted between April 2017 to August 2018. Patients age ranged from 2 months to 129 months and according to serum phosphorus level cases were classified into those with hypophosphatemia and others with normal phosphate level. The severity of illness was determined by using SOFA score. Results: Revealed that there was statistically significant decrease in phosphorus level at day 1 and day 3 between hypophosphatemia and normal phosphate level groups. The prevalence of hypophosphatemia was 46% at first day of admission and 70% at third day of admission. Significant increase of PTH and total bilirubin in hypophosphatemia group, and significant decrease of calcium in hypophosphatemia group compared to normophosphatemia group. The cases were followed and compared regarding; morbidity (need for mechanical ventilation, and length of PICU stay), degree of organ failure (SOFA score) and their outcome (discharge from PICU or death). Conclusion: It could be concluded that hypophosphatemia is considered a common co-morbidity in critically ill children in PICU. Hypophosphatemia more prevalent in those with respiratory problems, and higher SOFA score.