The aim of this work is to analyze the usefulness of rheumatoid factor determination in synovial fluid. One hundred twenty nine patients (29 with rheumatoid arthritis), in whom rheumatoid factor was simultaneously determined in serum and synovial fluid, were retrospectively analyzed. Serum rheumatoid factor had a 48% sensitivity, 98% specificity, 88% positive predictive value and 87% negative predictive value for the diagnosis of rheumatoid arthritis. These numbers were 76, 79, 51 and 92% respectively for synovial fluid rheumatoid factor. In rheumatoid arthritis of less than one year of evolution, serum and synovial rheumatoid factor have a sensitivity of 15% and 62% respectively, a positive predictive value of 50 and 28% respectively and a negative predictive value of 90 and 94% respectively. It is thus concluded that the absence of rheumatoid factor in serum and synovial fluid in a patient with arthritis of less than one year of evolution, renders the diagnosis of rheumatoid arthritis very unlikely. Likewise the simultaneous presence of rheumatoid factor in both fluids has a high diagnostic certainty. Among other studied variables, leukocyte count, C3 and C4 levels in synovial fluid are the best discriminators within the different diagnostic groups.