A patient with chronic Lyme arthritis and roentgenographic evidence of bony erosion underwent a synovectomy; proliferative synovium (pannus), containing aggregates of small lymphocytes, was found adherent to eroded cartilage and bone. During 8 days in tissue culture, the synovial cells produced large amounts of collagenase and prostaglandin E2, but only low levels of both neutral and acid proteinases. Sixty-seven percent of the lymphocytes from the synovium were T cells; 19% were B cells. Attempts to identify agent/antigen in the synovial cells were unsuccessful. Thus, the synovium of this patient, whose disease appears to be tick-transmitted, resembles that of rheumatoid arthritis. This finding further supports the hypothesis that many possible agents, including infectious ones, trigger a common pathway in synovium, which leads to joint destruction.