BackgroundTo determine whether undernutrition affects 60-day mortality in pediatric acute respiratory failure.MethodsSubjects with acute respiratory failure aged between two months and 13 years were included in the study. The Z-scores were calculated on admission and children were categorized into two groups of undernutrition and normal nutrition. The nutritional intake of the children was measured daily. The outcome was 60-day mortality.ResultsA total of 126 patients met the inclusion criteria; 41% were undernourished based on the Z-score of BMI and weight for height, 50% based on the Z-score of height and length for age and 45% based on the Z-score of weight for age. Overall, the 60-day mortality rate was 27.8%. The Cox regression analysis adjusted with PIM2, age and gender, showed that undernutrition has a significant relationship with 60-day mortality based on the weight for age Z-score (HR = 2.33; CI: 1.175–4.638). In addition, undernutrition has a significant relationship with 60-day mortality based on the BMI for age (HR = 3.04; CI:1.070–8.639) and weight for height (HR = 2.62; CI: 1.605–6.658) Z-scores. The mean calorie and protein intake of 72% of the children was less than 80% of their calorie needs. The time to start feeding in 63% of the children was more than 48 h. There was no relationship between the time of starting nutrition and nutritional intake during PICU admission and mortality.ConclusionUndernutrition is prevalent in mechanically ventilated children in the PICU and may be associated with 60-day mortality.