A 5-year-old boy trapped in a house fire was transported to the emergency department, unconscious with suspected carbon monoxide poisoning. The patient underwent a difficult intubation, but did not initially demonstrate any radiographic abnormalities. The patient remained intubated and underwent hyperbaric oxygen therapy using the carbon monoxide treatment protocol. Immediate post-therapy chest radiograph revealed the development of occult pneumomediastinum. The patient remained stable on positive-pressure ventilation and the pneumomediastinum resolved spontaneously. The patient was extubated on post-injury Day #2 and was discharged post-injury Day #4 with no residual clinical sequelae. The development of pneumomediastinum associated with hyperbaric oxygen therapy for carbon monoxide poisoning appears to be a rare phenomenon. However, clinicians should be aware of the risk factors that predispose patients to developing pneumomediastinum and have a low threshold for obtaining routine pre- and post-procedure screening chest radiographs in intubated and critically ill patients, particularly in children.