Ventilator-associated pneumonia (VAP) remains the most common nosocomial infection in intensive care units (ICUs). VAP occurs in 10-20% of patients who are mechanically ventilated for more than 48 h and carries a mortality of 10-50%. Procalcitonin is secreted as part of the systemic inflammatory response to infection. Serum values of procalcitonin vary greatly based on the type and severity of infection. Few studies have analyzed the behavior of procalcitonin in lung infection. However, the use of procalcitonin levels for the diagnosis of VAP has not been established. The aim of this work was to evaluate the clinical utility of serial measurement of Procalcitonin in the diagnosis and prognosis of ventilator associated pneumonia in mechanically ventilated children. Methods: TheSerial measurement of procalcitonin (PCT) and C-reactive protein (CRP) levels and the calculation of the simplified Sequential Organ Failure Assessment score (SOFA) and Clinical Pulmonary Infection Scores (CPIS) were performed in 40 patients mechanically-ventilated in pediatric ICU, Cairo hospital. In total, twelve VAP patients and twenty eight non VAP patients of similar age and sex were included in the study. PCT levels in serum samples were measured in all subjects. Results: Twelve patients (30%) developed ventilator associated pneumonia (VAP) and 28 patients (70%) didn't develop VAP. CRP level show no significant difference between the two groups (P=0.15).The SOFA tend to be higher in VAP group on Day 0 and become significantly higher on days 2, 4 (p=0.054, 0.001 and 0.001 respectively).The CPIS was significantly higher in VAP group on Day 2 (p=0.042). Mortality was significantly higher in VAP group (P=0.01). The area under curve (AUC) was (0.80) and (95%) Confidence Interval (0.61- 0.98) and p value (0.003), denoting a significant value at a cut-off 6 pg/ml (increased PCT by 6 pg/ml over basal value predict the development of VAP), with a sensitivity 75% and specificity 82%.In patients with confirmed VAP, procalcitonin levels were higher than in those with non-VAP (P value < 0.003).CRP level showed no statistically significant difference between those who developed VAP and those who didn't (P=0.15). Conclusions: The serial measurement of serum procalcitonin is a reliable marker for the diagnosis of ventilator-associated pneumonia and prediction of the ventilated patient's outcome. Combining this marker with the simplified Clinical Pulmonary Infection Scores and Sequential Organ Failure Assessment Score were significant tools for diagnosis and prognosis of ventilated patients.