Synovial biopsy of a knee joint was performed in 59 patients with arthritis of unknown cause. Light microscopy revealed a diagnosis in three. A cause became apparent in only a further 23 over a prolonged period of follow-up. Neither light microscopy, immunofluorescence nor electron microscopy discriminated between te principal diagnostic groups. Synovial fluid analysis was better than synovial biopsy at predicting osteoarthritis. Five men with giant effusions developed osteoarthritis and these might represent an entity. Synovial inflammatory changes were less pronounced in those with longer histories. Disease duration and activity may have more influence on synovial characteristics than the nature of the disorder.