To investigate the value of synovial fluid analysis in predicting the articular evolution of juvenile chronic arthritis, synovial fluid from 29 patients with oligoarticular onset juvenile chronic arthritis were examined prospectively. The patients were subsequently classified after a three year period of observation as having polyarticular (10 patients) or pauciarticular (19 patients) disease. The synovial fluid samples were analysed for total and differential white blood cell count, total protein, beta 2 microglobulin, and total complement activity. For comparison, synovial fluid samples from 95 patients with adult onset rheumatoid arthritis were also analysed. In patients with polyarticular disease polymorphonuclear cells and beta 2 microglobulin concentrations were higher than in the patients with pauciarticular disease (80 (29.2) v 58.1 (25.3), and 3.6 (1.2) v 2.2 (0.5) mg/l, respectively), but there was no significant difference from the patients with rheumatoid arthritis. Synovial fluid analysis may be useful in predicting the evolution of juvenile chronic arthritis and improve definition of its subtypes.