The differential diagnosis of subacute or chronic polyarthritic diseases, including rheumatoid arthritis, is based on recognizing a pattern of changes, with special emphasis placed on certain key features that possess higher specificity. The clinical history is by far the most important diagnostic tool and involves clear assessment of the distribution of joint involvement, whether pain is articular or extra-articular, whether a syndrome follows trauma or infection, and the duration of the process. The distribution of involved joints is a major contributor to differential diagnosis but can be misleading unless a skilled physical examination confirms objective synovitis. If present, nodules, which represent localization of the disease process, also offer a powerful tool in differential diagnosis. Rheumatoid nodules occur in about 20 percent of patients with well-developed rheumatoid arthritis. The effects of inflammation may be clinically detectable, but laboratory tests--e.g., the erythrocyte sedimentation rate and quantitative C-reactive protein--provide the most reliable evidence of inflammation. Synovial fluid analysis may reveal inflammatory changes and other abnormalities permitting diagnosis. Rheumatoid factor is present in about 80 percent of patients with rheumatoid arthritis but is not specific for this diagnosis.