Introduction: Recently malnutrition is considered one of the major problems nowadays in our community. Critically ill children have metabolic alterations and a catabolic state. Malnutrition is one of the factors that affect the outcome of critically ill children.Objectives: This study aimed to assess the accuracy of BMI and NRS (STRONGKIDS) in the assessment of malnutrition risk and the relation of malnutrition to the outcome of mechanically ventilated critically ill children. Methods: nutritional status using STRONGKIDs score and BMI of 101 mechanically ventilated children in pediatric intensive care units at Cairo University Pediatric Hospital was assessed. Outcome (mortality, hospital-acquired infections, and length of PICU stay and ventilation) was followed up to detect its correlation with malnutrition.Results: percentage of malnutrition depending on BMI Z-score was 39.6%, nutritional status depending on STRONG KIDS score showed a low risk of malnutrition 17.8%, moderate risk of malnutrition 65.4% and high risk of malnutrition was 16.8%. There was a significant correlation between STRONG KIDS score with BMI z –score with p-value 0.007. Also, there was a significant correlation between malnutrition (assessed by BMI and STRONGKIDS score) and HAI with p-value 0.05 and 0.035 respectively. There was also an inversely significant correlation between malnutrition (assessed by BMI) and a period of ventilation P-value 0.02.Conclusion: STRONG KIDS score is an easy method for assessment of nutritional status and can be used with BMI to avoid missing malnourished critically ill children as malnutrition is associated with prolonged mechanical ventilation and increase risk of HAI.