Purpose: Pneumomediastinum is rare in children and adolescents, and its causes have not yet been clearly determined. We aimed to identify the causes, clinical manifestations and prognosis of noniatrogenic pneumomediastinum in children. Methods: From February 2007 to June 2014, we retrospectively investigated 121 patients with pneumomediastinum under 18 years of age in 2 hospitals. Eighteen patients with pneumomediastinum after thoracotomy and 35 patients with iatrogenic pneumomediastinum were excluded. Results: Sixty-eight patients were divided into 4 age groups: those under 1 year of age (n=9, 13.2%), those 1 to 5 years of age (n=9, 13.2%), those 6 to 10 years of age (n=17, 25.0%) and those over 11 years of age (n=33, 48.5%). Chest pain (n=43, 63.2%) was the most common initial complaint and subcutaneous emphysema was identified in 18 patients (26.5%). Chest x-ray was diagnostic in all except 9 patients (13.2%). Predisposing causes of pneumomediastinum were idiopathic (n=26, 38.2%), respiratory tract infection (n=23, 33.8%), asthma exacerbation (n=4, 5.9%), trauma (n=4, 5.9%), endobronchial foreign body (n=2, 2.9%), interstitial lung disease (n=5, 7.4%), and neonatal respiratory disease (n=4, 5.9%). Chest pain (P<0.001) and idiopathic cause (P=0.001) were shown to linearly increase with age. On the contrary, tachypnea (P<0.001), dyspnea (P=0.016), and interstitial lung disease (P=0.008) were shown to have a decreasing linear association with age. The length of hospital stay was significantly increased in patients with interstitial lung disease (P=0.042), those with pneumothorax (P=0.044), and those without chest pain (P=0.013). Conclusion: According to age groups, there were significant differences in causes and clinical manifestations. In particular, pneumomediastinum that developed in younger patients with interstitial lung disease showed unfavorable outcomes, such as dyspnea, pneumothorax, and increased length of hospital stay. Therefore, precise evaluation of predisposing causes and careful management are needed for children with pneumomediastinum. (Allergy Asthma Respir Dis 2015;3:145-150)