The synovial membrane histologic sections from patients with six common rheumatic diseases were reviewed without knowledge of the clinical diagnosis. After histopathologic evaluation, the synovial membrane characteristics were grouped according to the patient's clinical diagnosis, and included 29 patients with rheumatoid arthritis, 13 with systemic lupus erythematosus, 17 with degenerative joint disease, 10 with acute bacterial arthritis, 8 with gout, and 13 with pseudogout. The only specific characteristics identified were bacteria (infectious arthritis), crystals (gout, pseudogout), and lymphoid follicles (rheumatoid arthritis). Nevertheless, other characteristic features of differential diagnostic utility were recognized, including the intensity and nature of synovial lining cell hyperplasia and of leukocyte infiltration. Light microscopic histopathologic changes in the common rheumatic diseases are not specific, but are of diagnostic utility. Complete and exhaustive review of each pathologic synovial membrane characteristic provides more justification for the routine use of synovial membrane biopsy as an adjunct to arthrocentesis in the evaluation of common rheumatic diseases.