Free air in the peritoneum is a portent of significant pathology in the patient with abdominal trauma. The finding of a pneumomediastinum (PM) on a thoracic computed tomography scan (CT) of a trauma patient is, however, not clinically well-defined. The objectives of this study were to evaluate the incidence, pattern, and outcome of CT-diagnosed PM in a cohort of injured patients. The trauma registry and radiology reports were reviewed retrospectively for all injured patients admitted over an 8-year period to determine the incidence of PM. Medical and radiological records of patients with a PM on thoracic CT were then reviewed to determine the pattern and outcome of the injuries. There were 1364 thoracic CTs performed in the study-period. The prevalence of PM was 5.2 per cent (71/1364). For the cohort of patients with a PM, the mean age was 34.8 years, and 14.7 per cent (10/68) had penetrating injuries. Of these 68 patients, 10.3 per cent (7/68) presented with nine clinically significant injuries to the esophagus, trachea, larynx, or bronchus. These injuries were suspected clinically by an associated open wound or significant symptoms, and only 5.8 per cent of (4/68) patients required surgical repair. The remaining 89.7 per cent (61/68) of patients with a PM did not develop any sequelae nor require further directed treatment. A finding of a pneumomediastinum on a thoracic CT in injured patients is rare and clinically nonspecific. Pneumomediastinum alone does not seem to be predictive of severe injury and warrants detailed investigation only when clinical symptoms are present.