We report the case of a 51-year-old woman who had suffered from right knee pain and stiffness for 40 years. Her symptoms had gradually worsened over the past 5 years. One year previously, when she first visited our clinic, plain radiographs and CT scan images had revealed synovial chondromatosis over the right knee. At the current admission, follow-up MRI showed synovial masses around the knee and worsening of endosteal cortical scalloping in the patella, femoral condyle, and tibial plateau. After diagnosis on the basis of frozen sections, the patient had total excision of the lesions and total knee athroplasty. Histological examination revealed synovial chondromatosis in the joint cavity and grade 1 chondrosarcoma invasion into the adjacent bone. In long-standing synovial chondromatosis, presentation with aggravated symptoms and deterioration on imaging findings should alert clinicians to the potential for malignant change.