The objective of this study was to determine: (1) the diagnostic value of antiperinuclear factor (APF), (2) the types of immunoglobulins involved in the reaction and (3) the presence of the antibody in paired samples of serum and synovial fluid (SF). We studied 408 serum samples from the following: healthy controls (n = 68), patients with rheumatoid arthritis RA; n = 160, 106 RF-positive and 54 RF-negative and patients with other rheumatic diseases (n = 180). We examined paired serum and SF samples in 27 patients (8 with RA and 19 with other rheumatic conditions). APF was determined by an indirect immunofluorescence assay. A group of 30 APF-positive serum samples was incubated with fluorescent-labelled antisera against IgG, IgM and IgA independently. APF was positive in 55.7% of patients with RF-positive RA, in 35.2% of patients with RF-negative RA, in 11.1% of patients with other rheumatic diseases and in 5.9% of healthy controls. Statistical differences were found between RF-positive RA and the other three groups (P = 0.02, P = 0.0001, P = 0.0001, respectively) and between RF-negative RA and the groups of other rheumatic diseases (P = 0.0001) and healthy controls (P = 0.005). The specificity of the test for RA was 90.2%. APF was present in three SF samples from RA patients (37.5%). The reaction was mediated by immunoglobulins of the IgG class in 100% of those tested, and, in addition, 30% were of IgA and 6.7% of IgM classes. We concluded that APF is a good diagnostic test that could be included in the classification criteria of RA, it can be present in SF and it is predominantly an antibody of the IgG class.