The authors present three patients who developed a tension pneumothorax while receiving emergent hyperbaric oxygen therapy for acute carbon monoxide poisoning. Each patient was intubated and received closed chest compressions for cardiac arrest prior to hyperbaric oxygenation. Despite the apparent absence of pneumothorax prior to hyperbaric therapy, tension pneumothorax was detected soon after decompression. These cases illustrate the need for vigilance in detecting and addressing pneumothorax prior to hyperbaric decompression in obtunded patients. Serial physical examinations, arterial blood gas determinations, properly positioned chest radiographs, and a high index of suspicion for pneumothorax in the setting of emergent hyperbaric therapy are recommended.