A previously healthy 33-year-old man presented to the emergency department with a 10-day history of lethargy and feeling unwell. His temperature was 38.3°C, and he had tachycardia, with an oxygen saturation of 85% while breathing 15 liters of oxygen per minute through a nonrebreather mask. The physical examination was notable for bronchial breath sounds in the left lower lung; no subcutaneous emphysema was palpated. Chest radiography showed perihilar opacities in both lungs with free air in the mediastinum (Panel A, arrows). Computed tomography confirmed pneumomediastinum (Panel B, arrows) with enhanced interstitial markings and lower-lobe opacities in both lungs, with greater severity in the left lung. The throat swab was positive for influenza A (H1N1). The patient received noninvasive ventilatory support and was treated with zanamivir for influenza and piperacillin–tazobactam for bacterial superinfection. The pneumomediastinum resolved without additional intervention, and the patient was discharged from the hospital 15 days after presentation.