Paired synovial fluid (SF) samples obtained from the knees of 12 arthritis patients were studied to establish a relation between parameters of local inflammatory activity and SF interleukin-6 (IL-6) levels. Local disease activity was scored using joint temperature, swelling and pain as clinical parameters of inflammation. SF samples were assayed for laboratory parameters of inflammation such as leucocyte content, the percentage polymorphonuclear cells, the pH, and for immunoglobulin levels (IgG, IgM). SF IL-6 concentrations were determined using the B9-bioassay. Within individual patients the local activity of inflammation as measured using clinical parameters was found to be related to the local SF IL-6 level. When considering the total group of patients, a correlation (P less than 0.001) was found between the clinical parameters of local inflammation and the SF IL-6 levels. Furthermore, IL-6 levels were found to correlate with leucocyte counts (P less than 0.02), the percentage of polymorphonuclear cells (P less than 0.10), the pH value (P less than 0.01), but not with SF IgM and IgG concentrations.