We studied 10 patients who had arthritis of the knee joint, but no other signs of rheumatic disease. The clinical diagnosis of osteoarthritis was corroborated by arthroscopic evidence of characteristic cartilage degeneration. Signs of inflammation were confined to areas of the synovial membrane that lay near the cartilage; thus, the major part of the joint cavity was not affected. The intensity of the synovial inflammation varied within the areas involved, but was always most pronounced in regions rimming the cartilage. Biopsy samples selected from regions of intensely inflamed synovium contained foci of T lymphocytes, which were bordered by immunoglobulin-carrying B lymphocytes and plasma cells, as well as strongly HLA-DR positive dendritic-like cells adjoined to alpha Leu-3a+ T helper lymphocytes. In tissue samples taken from macroscopically noninflamed areas, only a few infiltrating lymphocytes were seen. Thus, the inflammatory synovial changes found in osteoarthritis appear to be anatomically restricted and of varied intensity but, when present, are microscopically indistinguishable from the changes that have been previously described as indicative of rheumatoid arthritis.