Results of synovianalysis are reported in 129 children, 91 with juvenile rheumatoid arthritis (JRA), 13 with septic arthritis, 12 with enteroarthritis, 12 with acute transient arthritis and 1 with bacillus Calmette-Guerin arthritis. Mononuclear cells were dominant in the patients with oligoarticular JRA (mean 64%, median 74%), and among these, significantly lower proportions of polymorphonuclear (PMN) cells were found in patients with early onset disease with antinuclear antibodies or chronic uveitis than in those with late onset with HLA-B27. PMN cells were dominant in polyarticular and systemic onset JRA, septic arthritis and enteroarthritis. The sensitivity and specificity of a white cell count above 40,000/mm3 exceeded 90% in differentiating septic arthritis from the other kinds of arthritis. Measurement of total protein, glucose and lactate in synovial fluid was of limited diagnostic value.