Background: Pneumothorax should be considered a medical emergency and requires a high index of suspicion and prompt recognition and intervention. Aim of the work: was to study the prevalence and risk factors of pneumothorax among children in Tanta PICU. Subjects and Methods: Sixty pediatric patients aged from 2 to 180 months (29 males and 31 females) admitted to PICU, Tanta University Hospital and were divided into 3 groups: Group 1: 20 patients on PCMV. Group II: 20 patients on HFOV: Group III: 20 non-ventilated patients. Each group was further divided into two subgroups: Subgroup I (SGI): without pneumothorax, Subgroup II (SGII): with pneumothorax. All patients were subjected to scoring systems for Pediatric Risk for Mortality (PRISM III) and Sequential Organ Failure Assessment (SOFA). They were also monitored for [pulse oximetry, blood pressure, oxygenation index, oxygenation saturation index, lung mechanics (compliance and resistance), ventilation parameters (HFOV and P-CMV) and trans-esophageal Doppler]. Results: Incidence of pneumothorax in patients on P-CMV is higher than who on HFOV and non- ventilated group. The significant risk factors of pneumothorax were HR, RR, pH, PCO2, HCO3, PaO2, SaO2, OI, MAP, FIO2, PIP, CO, CI, SVRI, SOFA and PRISM III score. X-ray showed equal results to CT chest in the diagnosis of pneumothorax without the disadvantage of exposure to high radiation accompanied the use of CT. Conclusion: Pneumothorax in P-CMV patients occur with higher ventilatory settings. Most cases of pneumothorax have underlying lung disease as pneumonia and ARDS. Mortality rate is higher among patients with pneumothorax.