Twenty patients having generalized osteoarthritis (GOA) and symptomatic temporomandibular joints (TMJs) were compared with 22 patients having rheumatoid arthritis (RA) and TMJ symptoms, and also with an age-matched reference tissue material obtained at autopsy from 17 TMJs. Muscle tenderness was commoner in GOA. Arthroscopically, high frequencies of synovitis, degenerative changes, and fibrosis were observed in both groups, with more pronounced inflammatory and degenerative changes in RA patients, despite a shorter duration of TMJ symptoms. A correlation was noted between lateral joint tenderness and pronounced synovitis in RA patients. Histologic and immunohistochemical examinations added useful information to arthroscopy and showed similarly high frequencies of synovial inflammation in GOA and RA patients, differing clearly from those in the reference material. Connective-tissue degeneration was commoner in GOA patients. GOA and RA probably have different causes, but, interestingly, the tissue reaction was similar in the TMJs, although pronounced inflammatory and degenerative changes seemed to develop faster in RA.