Background: Hemothorax is a rare complication of hereditary multiple exostosis.

Methods: A 12 year-old boy suffered from abrupt thoracic pain, firstly attributed to pleural effusion. He had hereditary multiple exostosis known since the age of 9 years. The patients was given anti-inflammatory drugs and erythromycin but the pleural effusion became more abundant 6 days later requiring thoracentesis which showed hemothorax. All bacteriological and cytologic investigations were negative. X rays, ultra-sonography and CT scan showed several costal exostoses developing into the thoracic cavity. The hemothorax disappeared within 12 days and the patient was well 4 months later, without pleural sequelae.

Conclusions: Hemothorax may be due to internal costal exostosis. It may be cured with thoracentesis; more aggressive therapy should be performed in exceptional cases with severe and/or recurrent bleeding.