To determine the clinical significance of the microscopic study of the synovial fluid (SF) sediment, 306 sediments were examined from 216 patients with the most frequently occurring acute and chronic arthropathies. The SF was obtained using a fine gauge needle. Microscopic rice bodies were seen in 53 (17.3%) of the samples studied. Forty-five (84.9%) of the samples in which rice bodies were observed were from patients with rheumatoid arthritis (RA) (p < 0.001); 34.9% of the 129 RA samples were found to contain rice bodies. The specificity of finding rice bodies in RA was 95.5%. Most rice bodies were composed of partly or totally hyalinized fibrinous material; in their interior, mononuclear cells were predominantly observed--most of them macrophagic in appearance. With less frequency the rice bodies exhibited central areas of fibrosis. Our results suggest that microscopic rice bodies are fibrinous particles derived from the entrapping of cells in the dense fibrin network present in inflammatory SF.