Polyarthritis and tenosynovitis were seen as the presenting complaints in 7 patients with secondary syphilis. Synovial needle biopsy was obtained in 3 cases and showed mild synovitis with infiltration by lymphocytes, plasma cells, and polymorphonuclear leukocytes; vascular congestion; and slightly increased synovial lining cells. In addition, electron microscopy in these synovial membranes showed many necrotic polymorphonuclear leukocytes, much interstitial debris and fibrin, and in 2 patients, bodies suggestive of Treponema pallidum in the interstitium and in vessel lumens.Articular and bone diseases have only occasionally been described as prominent manifestations of secondary syphilis (1-3). Since no modern studies attempting to identify spirochetes in synovial fluid or membrane have been performed and biologic false positive serologic reactions for syphilis can be seen with From the University of Chile, Santiago, Chile, and the Veter-