Synovial cell sarcoma is the most common malignant tumor involving the joints. The tumor arises from primitive mesenchymal cells outside the synovial membrane. Clinical signs include lameness and joint swelling. Radiography reveals soft-tissue swelling and a periosteal reaction. Pulmonary metastasis is detected in ~25% of animals at initial examination. Biopsy reveals evidence of a soft-tissue tumor. Limb amputation is the treatment of choice, although palliative radiation may be considered for cases with a low tumor burden not involving bone.