We present a diagnostic course and treatment of synovial sarcoma in a young 16-year-old male patient. The clinical course with special emphasis on the imaging techniques is presented here, providing an overview of this subject and offering a useful educative material. The course of the follow-up is also described, largely relying on MRI in diagnosis course. Changes in the synovial tissue of the knee were recorded, later to be classified as the sarcoma. Post-operative course suggested the existence of chronic synovitis. Further studies are needed to fully understand the changes that may affect knee mechanics after surgery and/or inflammation factors secreted by tumor cells.