Hereditary multiple exostosis is a rare genetic disease. Its severity is related to the risk of malignant degeneration. We report a case of a 22-yeasr-old man who consult for a mechanical gonalgia. He reported the presence of a multiple asymptomatic bone outgrowths, evolving since the age of 13, located in the limbs and the thorax. The X-rays showed typical images of exostosis. Through this clinical case, we propose a new element of prognosis based on the analysis of the patient's clinical data and a family survey.