The authors examined 487 synovial fluid specimens in patients affected with spontaneous effusion in the knee. In 202 of the cases the features indicating the nature of the effusion were observed (rheumatoid arthritis: 27; active S.L.E.: 2; microcrystals: 77; infections: 38; doubts as to rheumatoid arthritis or other connectivitis: 58). In 84 cases no features were determined despite the presence of specific clinical signs (psoriasis, arthrosis, previous trauma). In 201 cases where there were no clinical signs in the synovial fluid, the authors were able to differentiate moderate phlogosis in 27 specimens, and intense phlogosis in 104. Based on the results obtained, an attempt was made to define which tests are best to measure the amount of inflammation and which may be correlated with the etiology of the joint effusion.