Tehran University of Medical Sciences Acta Medica Iranica 0044-6025 55 12 2018 01 16 793 799 EN Khodamorad Jamshidi Bone and Joint Reconstruction Research Center, Shafa Orthopedic Hospital, Iran University of Medical Sciences, Tehran, Iran. Tina Shooshtarizadeh Bone and Joint Reconstruction Research Center, Shafa Orthopedic Hospital, Iran University of Medical Sciences, Tehran, Iran. Mehrdad Bahrabadi Bone and Joint Reconstruction Research Center, Shafa Orthopedic Hospital, Iran University of Medical Sciences, Tehran, Iran. 2016 08 23 2017 02 08 Metachondromatosis which was first described in 1971 by Maroteaux is a rare genetic disease consisting of osteochondromas and enchondromas, caused by loss of function of the PTPN11 gene. It is distinct from other cartilaginous tumors such as multiple osteochondromas and hereditary multiple exostosis by the distribution and orientation of lesions, and pattern of inheritance. In Metachondromatosis osteochondromas typically occur in hands, feet, femur, and tibia while enchondromas commonly affect the pelvic bones and femurs. Both tumors are generally reported to regress in adulthood. To the best of our knowledge only one case of Chondrosarcoma has been reported, and our case is the second reported case of Chondrosarcoma in metachondromatosis. https://acta.tums.ac.ir/index.php/acta/article/view/5832 https://acta.tums.ac.ir/index.php/acta/article/download/5832/5033