The spectrum of immunohistological change in the affected joints of patients with rheumatoid arthritis has been well described. In this study, the immunohistological features in synovial membrane obtained from apparently uninvolved knee joints of 16 patients with active untreated rheumatoid arthritis were examined and compared to tissue from control subjects. Synovial tissue was obtained by needle biopsy. Hyperplasia of the synovial lining layer, present in 69%, was the most frequently observed abnormality in synovium obtained from uninvolved joints. Perivascular mononuclear cell infiltration was present in 31% and consisted predominantly of helper T-cells. Increased vascularity and fibrin deposition were not notable features. Clinically overt synovitis emerged in only two patients during a follow-up period of up to 36 months. In conclusion, a considerable degree of histological change was observed in the apparently uninvolved knee joints of patients with active rheumatoid arthritis. The presence of subclinical synovitis challenges current concepts of disease activity and clinical remission. Further study is required to determine whether the features described may be associated with progressive joint erosion.