Objective: Zonulin is a biomarker of increased intestinal permeability. Malnutrition is strongly associated with prolonged length of stay, increased infection, and mortality. This study aimed to determine how the serum zonulin level is affected in patients in PICU, and evaluate the relationship between pre-albumin and Vitamin D. Method: 35 critically ill pediatric patients were included in the study. The control group was formed of 25 healthy children. The albumin, pre-albumin, Vitamin D, and zonulin levels were examined in patients with findings of infection that regressed during ICU follow-up. The differences between prealbumin levels, Vitamin D, and zonulin were analyzed with the Mann-Whitney-U Test. Results: The most common reason for admission to the PICU was respiratory failure in 12 patients. The Vitamin D level was determined to have a mean of 28.8±12.3ng/mL and <29ng/mL in 9 patients. The albumin level was determined to be mean 3.3±0.6 mg/dL, and the pre-albumin level was mean 17.8±7.4mg/dL. The serum zonulin levels in critically ill patients were statistically significantly higher than in the control group. The difference between the zonulin levels of patients with pre-albumin values was statistically significant. The zonulin levels of patients with a Vitamin D value <20 were found to be statistically significantly higher than the zonulin levels of patients with a Vitamin D value >20 Conclusion: The higher zonulin level in critically ill pediatric patients may be associated with prolonged catabolic processes, exposure to oxidative and hypoxic stress, and bacterial translocation development associated with all of these. The results of the current study showed a statistically significant negative correlation between Vitamin D and zonulin levels. Therefore, the relationship between low Vitamin D values and a high zonulin level may be useful in evaluating chronic malnutrition. The serum zonulin level selected as a biomarker for the surveillance and management of nutrition in critically ill pediatric patients is not an appropriate marker.