We examined sections of synovial membranes from 14 patients with rheumatoid arthritis (RA), 7 with other rheumatic diseases, and 10 with no apparent joint disease. Patients with RA and other rheumatic diseases had significantly more synovial mast cells/vessel than patients with no joint disease (0.49 and 0.20, respectively, versus 0.03). They also had significantly more total mast cells/10 fields than patients with no joint disease (9.9 and 5.0, respectively, versus 0.4). Within the rheumatoid group, patients with active disease had more total mast cells/10 fields than patients clinically considered to have end-stage disease (P less than 0.05). Synovial basophils were not identified in any patient. Synovial vascularity was similar for all groups (2.3 vessels/field). The role of the synovial mast cell in RA and other rheumatic diseases remains to be determined.