Synovial chondromatosis is a rare condition in which osteocartilaginous nodules are formed by synovial metaplasia and become intra-articular loose bodies. It is usually monarticular, most commonly affecting the knee and is regarded as invariably benign. There are reports of malignant change, but only the two case studies of Mullins, Berard and Eisenberg (1965) and of Dunn et al. (1974) fully document the development of chondrosarcoma in this condition. We report a patient with a long history of histologically proven synovial chondromatosis in whom the condition became clinically aggressive and underwent malignant transformation to chondrosarcoma.