RESUMEN
Synovial chondromatosis is a rare disease. It occurs as primary or secondary lesion with involvement of joint, tendon and bursa [extra articular] or combined. We present a case of subacromial chondromatosis with presentation of a large cystic lesion with rapid growth and containing numerous chondral particles. The patient was a 16 year old boy admitted with a huge mass at right shoulder which was begun since 3 months. Sonography and CT scan showed a mass of 7 x 10 x 4 cm, which contained small particles. The patient was operated and a very large cystic mass with numerous rice bodies in it was removed. Pathologic examinations revealed primary synovial chondromatosis. Synovial chondromatosis should be considered in differential diagnosis of cystic masses around the joints especially shoulder