Background: Critical illnesses caused by any severe medical or surgical diseases may trigger an acute phase response which is associated with several metabolic derangements. These include hypo- and hyperglycemia, hypo-and hypercalcemia, hypo-and hyperphosphatemia, hypo-and hypermagnesaemia. Objectives: To find out the incidence of calcium and magnesium disturbances in critically ill children and to evaluate their correlation with severity of illness, complications, mortality and length of hospital stay.Methods: A prospective observational study included patients admitted in the pediatric intensive care unit (PICU), Cairo University Pediatric Hospital, during the period from May 2012 to January 2013 for electrolyte screening on admission and every three days till ten days of admission.Results: The incidence of different electrolyte disturbance at admission to PICU was: hypophosphatemia (47.1%); hypocalcaemia (34.3 %); hypomagnesaemia (31.4 %); hypermagnesemia (7.1%) ; hypercalcaemia (5.7%); hyperphosphatemia were (2.9%). Patients with acidosis and hyperglycemia had lower calcium level. Patients with in born error of metabolism and hypophosphatemia had lower magnesium level. Overall Mortality rate was 37.1% but we did not find any correlation between electrolyte disturbance and mortality, but we found that patients who were ventilated and treated with Amphotrecin B had a prolonged length of stay.Conclusion: Metabolic disturbances in critically ill children are frequent findings. Hypophosphatemia was the commonest electrolyte disturbance followed by hypocalcaemia in our study. Thus, regular controls of different metabolites must be performed so we can precociously detect these disturbances and correct them.