We measured serum and synovial fluid interleukin 6 (IL-6) levels in patients with polyarticular-onset and pauciarticular-onset juvenile chronic arthritis (JCA), using the hybridoma cell line B9. During active disease, but not during remission, serum IL-6 levels were significantly elevated in patients with polyarticular (p < 0.0001 vs controls) and in patients with pauciarticular JCA (p < 0.01 vs controls). In patients with active polyarticular disease (polyarticular and extended pauciarticular) serum IL-6 levels correlated with the extent and severity of joint involvement (p < 0.05), with erythrocyte sedimentation rate values (p < 0.005) and with C reactive protein concentrations (p < 0.05). In patients with persistent pauciarticular JCA, serum IL-6 levels correlated with the joint swelling score (p < 0.05). Synovial fluid IL-6 levels, measured in 5 patients with pauciarticular JCA, were markedly elevated (range 600-24,000 HGF U/ml). In conclusion, our data suggest that IL-6 is an important pathogenic mediator in polyarticular and pauciarticular JCA.