The consequences of an undiagnosed pneumothorax can be life-threatening, particularly in patients with trauma to the head or multiple injury and in those requiring mechanical ventilation. Yet it is these patients, whose films will be assessed initially by the surgeon, who are more likely to have a chest X-ray taken in the supine position. The features of supine pneumothoraces are described and discussed together with radiological techniques used to confirm the diagnosis, including computed tomography (CT) which may be of particular importance in patients with associated cranial trauma.