In 17 patients with Lyme disease, synovial specimens, obtained by synovectomy or needle biopsy, showed nonspecific villous hypertrophy, synovial cell hyperplasia, prominent microvasculature, lymphoplasmacellular infiltration, and sometimes lymphoid follicles. The larger surgically obtained specimens also showed striking deposition of fibrin in synovial stroma and a form of endarteritis obliterans. In 2 patients, spirochetes were seen in and around blood vessels by the Dieterle silver stain. Compared with 55 cases of other synovial disease, obliterative microvascular lesions were seen only in Lyme synovia, but marked stromal deposition of fibrin seemed nonspecific. These findings imply that the Lyme spirochete may survive for years in affected synovium and may be directly responsible for the microvascular injury.