Synovial biopsies were obtained from 28 patients with various kinds of chronic arthritis, at synovectomy and 6 and 12 months later. The tissues were examined by immunofluorescence technique, recording the quantities of cells and extracellular deposits staining with polyclonal antisera to IgG, IgA, IgM, C3c, fibrinogen, and chi and lambda light chains, and monoclonal antibodies to CD3, CD5, CD11b, HLA DR, and TCC (Terminal Complement Complex). These parameters were compared with scores obtained by arthroscopy and clinical evaluation (Colorado Knee Score) performed at the same time. Taken as a group, the immunological parameters showed reduction in activity 6 months after synovectomy (p less than 0.01), and a tendency to revert to base-line values after 12 months. A similar reduction in activity after 6 months was also found by arthroscopic and clinical evaluation. Thus, this longitudinal study demonstrated a relationship between changes in immunologic activity, arthroscopic findings and clinical activity after synovectomy in patients with chronic arthritis. This is consistent with an immunological pathogenesis for the inflammation in these joints.