We present the case of a male adolescent with a haemothorax caused by pericardial rupture secondary to costal exostosis. The patient was affected by multiple congenital exostosis and six years earlier he underwent a bone spur resection of the right femur. An X-ray performed at presentation was interpreted as suggestive of pneumonia and the patient was therefore treated with antibiotics without any clinical improvement. A thoracic ultrasonography performed 7 days later showed a massive haemothorax, confirmed by a CT scan on which a pericardial lesion caused by the growth of three osteo-cartilagineous spurs originating from the 5th, 7th and 8th rib was detected. The boy underwent surgery and recovered completely. The thoracic ultrasound and the subsequent CT-scan performed on the patient due to his lack of clinical improvement directed us towards a rare disease that the X-ray had not helped diagnose; this entailed the prompt surgical resolution of a possibly severe, maybe even fatal, condition.