We describe a new clinical sign in a case series of three patients who developed pneumothoraces during mechanical ventilation in the intensive care unit. All three patients were in the supine position. Two patients had x-rays that were inconclusive before insertion of chest drains and the third had a pneumothorax diagnosed on clinical findings alone. On each occasion we were able to diagnose pneumothorax using sternal percussion and simultaneous auscultation. The method relies on percussion of the sternum while simultaneously ausculating the anterior (superior) chest on the side of the suspected pneumothorax. The stethoscope is then placed on the other side of the chest. The percussion sound on the affected side has an exaggerated, resonant and booming quality. The percussion note is exaggerated partly because a stethoscope is used and partly because, in the supine patient, air localizes upwards to the anterior thorax.