A post-traumatic pulmonary pseudocyst is a rare complication of blunt chest injury, in which an increase in intrapulmonary pressure causes shearing of the lung parenchyma without rupture of the pleura. We describe a young man who presented with a history of haemoptysis and a cavitary lesion on his chest film two weeks after a blunt chest injury due to a fall. Computed tomography showed appearances consistent with a post-traumatic pseudocyst, and bronchoscopy disclosed evidence of inflammation, with growth of abnormal flora in the bronchial washings. Administration of oral antibiotics based on sensitivity testing led to rapid and complete clinical and radiological resolution. Pneumon 2005, 18(2):228-232.