Synovial sarcomas are unique and relatively rare malignant neoplasms frequently arising near, but not necessarily from, the synovium of joint capsules, bursae, or tendon sheaths. This paper presents the clinical and roentgenologic findings in 24 cases of this tumor observed at the Mayo Clinic. Background Information The earliest reports of synovial sarcoma are generally accredited to Stüer in 1893 (1), to Salter and Hardie in 1894 (2), and to Lockwood and Turner, both in 1902 (3). Knox (4) credits v. Ruediger Rydygier with presenting the first synovial sarcoma in 1906. Pack and Ariel (5) accept Lejars and Rubens-Duval's report in 1910 as the first adequate presentation of these tumors. In 1927, Smith (6) suggested the name “synovioma” for such tumors and described their clinical and pathologic aspects. Knox in 1936 and Berger (7) in 1938 reviewed the cases reported in the literature and described new cases of their own. Both writers emphasized the histopathology of synoviomas. Excellent articles by Bennett in 1947 (8) and by Pack and Ariel in 1950 described these neoplasms in detail. Summaries of the cases reported in the literature were presented by Haagensen and Stout in 1944 (9) and by Tillotson, McDonald, and Janes in 1951 (10). In each paper new cases were added and a complete analysis of all cases previously reported was presented. Articles dealing with the roentgen manifestations of synovial sarcomas have not been numerous. Lewis in 1940 (11) presented his findings in 4 cases. Others (5, 12–14) have briefly described the roentgenograms in some of their cases. Recently papers by Hale (15) and by Sherman and Chu (16) have more closely analyzed the roentgen features. Pathology: The normal synovial membrane consists of an intimal lining and a supportive layer of connective tissue. According to Bennett, this lining membrane of specialized mesenchymal cells secretes mucin and regulates the passage of substances between the articular and vascular fluids in the joints. The intimal lining may appear as a single layer of flattened cells or may increase to many layers of round, oval, or cuboidal cells. When this occurs, papillary projections with recesses and crypts may be noted (8). The normal connective-tissue layer frequently is characterized by highly vascularized zones and may be loose or compact. Since the term “synovioma” has frequently been used to include both benign and malignant neoplasms arising from synovial membranes, it is probably more correct to use the term “synovial sarcoma” for the tumors considered here. Such neoplasms present both a synovial and a fibrosarcomatous element (10). The relative amount and degree of differentiation of each element may show wide variation. The fibrosarcomatous element produces reticulin, which can be demonstrated with special silver stains. Synovial sarcomas grow by expansion and are frequently circumscribed. Some are surrounded, completely or in part, by a pseudo-capsule.